Can I transfer Time Machine data from two separate hard drives into one new one?

Question

I'm using a MacBook Pro as my primary computer.  My 500 gig Time Capsule filled up a year or so ago, so I stopped using it with Time Machine for awhile so I could keep the data from those old back-ups.  There were a number of things I deleted from my computer's very limited hard drive after they were backed up to the Time Capsule.  I got a 1T external USB drive last year to use as my "filing Cabinet" to store files I didn't necessarily need all the time or that were filling up my small laptop hard drive--including my iTunes library--all organized in a way that made it relatively easy for me to find what I needed, even if I didn't remember exactly when I'd filed it or what I'd called it.  I got another 1 terabyte external (portable) drive last July and dedicated it to TimeMachine backups and labeled it "TimeMachine".
Over the last couple of weeks, my friend has been helping me upgrade to Yosemite and clean up my laptop hard drive.  Last week he cloned my laptop hard drive to a new 1T hard drive and I exchanged it for my old drive in my computer today. All good.
Here's the issue.  We replaced my Time Capsule hard drive with a 1 terabyte drive with the idea of transferring the data from the old Time Capsule (500G) drive and the newer USB 1T "TimeMachine" compact drive to the new 1T Time Capsule drive and beginning using the latter for my Time Machine backups going forward.  Originally he thought we could copy everything from each of my external drives (the old 500gig drive from my Time Capsule, the USB "TimeMachine" drive I've been using since July, and the "file cabinet" files) to my computer in their own folders and then start regular TimeMachine backups to the new Time Capsule drive, thus preserving all my old data and making regular backups going forward.  The "file cabinet" data was no problem at all, but when I tried to copy my USB "TimeMachine" data to my computer, I was unable to.  My friend found instructions for transferring old TimeMachine data to a new TimeCapsule, but I don't know if I can transfer the data from two separate disks to the new TimeCapsule drive. I'm afraid that one set of data will supersede the other and either my newer backups or my old ones will be lost if I try to transfer both. 
Are my fears justified or is there a way to insure that no such problem will occur?  Of course, my data will still be on those two older drives, but that won't do me a lot of good if I can't access it when I need to. Also, the 1T drive now belongs to my friend; he used a brand new drive he'd bought for himself for my new internal hard drive and plans to take my 1T "TimeCapsule" drive in exchange, once the data has been transferred, so he will, of course, erase that drive. 

Answer 1

Should we be able to bring up the old (500G) Time Capsule Drive to rename it using a SATA to FireWire harness and then copy the whole thing to the new Time Capsule drive?
You can copy a whole sparsebundle from one drive to another. That is not a problem. Whether you can access the sparsebundle is something you should test before you even start though.
If it's on the Desktop and I don't tell Time Machine to exclude it from backups, will it just automatically back it up?
All drives you plug into the Mac are excluded by default.. you must include them. So no problems there.. but I hope I am understanding the question.
Will both volumes or directories (which is the right term?) show up when I open Time Machine?
No, Time Machine will only open what it is told to open... or its backup default location.
You can force Time Machine to open alternative/old/no longer used backups by (now I have a problem as things have changed somewhat in Yosemite and I consider it alpha release software at this point in time). The old method was to right click on the TM icon and select a different TM backup. easy. Yosemite seems to have made easy stuff harder.
Here it is on my current computer.. clearly not Yosemite.. Right click on the TM icon in the dock.. select Browse Other Time Machine Disks.. And supply the info of where that is located. Easy. If you cannot figure it out one of the other posters here (with more patience than me for Yosemite) will help you.
Or will we have to partition the new drive somehow--is that even possible?
I am getting more lost as I go down the list.. but the TC disk cannot be partitioned.
If you have included all those USB drives in the new backup on a Time a Time Capsule.. you have made life rather hard because now your files are stored another layer deeper than they were.
So to open a file from a disk you need to open the sparsebundle.. then dig down to the drive in question and then dig down to the backup.. and all of this means Time Machine has to work perfectly which is Yosemite is a very big ask.
I thought you wanted to just backup your old drives to central location.. which means copying the files to a separate folder on the Time Capsule.
One correction I need to make to my post, which will make my strategy make a bit more sense: my new Time Capsule drive is 4T
It makes it much harder.. and I have to pose a real question of long term .. if you have put a 4TB drive in a Gen1 TC.. did you also replace its power supply because I can assure you the drive might be ok but the TC itself will not last forever.. and what happens when it dies. The Gen1 power supply is already well beyond its normal life span and the vast majority are dead. When the backup device is unreliable and the backups on it are made that much harder to get access to.. is this a great plan??
If you are going to consolidate all your old files on one disk.. a task I find understandable. I have done much the same albeit the usefulness of files made on emac running OS9 may be questioned. A disk lying in the bottom of a draw is a more appropriate place for them.
You want those files as easily accessible as possible (at the point of recovery) and not buried inside a sparsebundle.. particularly not a sparsebundle from the old TC disk buried inside a new sparsebundle.. keep files as accessible as possible as you can run searches.. and that is best done on a USB 3 (or faster ie thunderbolt) drive plugged into a new(ish) computer.. not network. And since the files are not being accessed on a daily/weekly or even monthly or yearly basis.. keeping them in actively running TC network storage.. I would say is a waste of space. That is only my opinion of course.. you might consider it highly important that files you will never look at are available any time of day or night when the urge comes to track down that elusive pimpernel email you sent 10 years ago... but I find it hard to justify. What the case.. the problem with TM and things like Mail is you cannot search it.. you must restore the whole library/files/program even before you can access it.. that makes file recovery out of a sparsebundle double step process.
So.. summary.
If you want to store files on the 4TB drive in the TC.. that is not a great strategy but it can work.. simply create a folder named.. OldFilesEMac for instance.. and do a simple copy and paste of all the file to that location. Do not use TM.. Since you have already used TM.. and from what I am reading you have already done the backup with the external drives included.. then you are going to end up needing to erase the TC and start over.. which you may not be prepared to do.. which is fair enough. (I am coming across as overbearing school master.. apologies).
TM is to backup your main OS and current files.. not files from 10-20years ago.
Please do read the issues involved in Pondini..
See his FAQ. I recommend you read through Q14-17 so you understand what is involved in recovery.
http://pondini.org/TM/FAQ.html
I also recommend you read the first couple of articles here. http://pondini.org/TM/Home.html
Particularly so you understand the complexity of Time Machine.
And the articles here. http://pondini.org/TM/Time_Capsule.html
Particularly Q3 on mixing data and backups on a TC.
I wish I could spend an hour or two face to face and work it out.. the whole strategy to do this.. !!