My time capsule is getting full. What are my options?

Question

I have a 1TB Time Capsule for the MacBook Pros of myself and my wife.  One MBP currently has ~420 GB on it (750 GB HD) and the other currently has ~180 GB on it (500 GB HD).  Nothing else is backing up to this Time Capsule, and no other files are stored on it. So my total HD data to be backed up is ~600 GB.
I have been getting "The backup disc is full" warning for some time on the larger MB.  It reports that older backups are being deleted and gives the date of the oldest backup.  Just a few weeks ago, that was a couple of months back.  But lately (been doing a lot of work on this machine) the "oldest" date has been coming closer to the present at an increasing rate.  Today the oldest backup is down to about 10 days in the past!
The Time Machine Preferences dialog informs me that there is 1.47 GB reamining available of 998 GB (nominal 1 TB size of the drive).  The other MBP is used MUCH less (by my wife) but it has had the same messages periodically, though her oldest b/u date is much further back still.  Time Machine Prefs report the same 1.47 GB available of the same 998 GB.
So, not entirely understanding how Time Machine apportions space when two computers are using the same Time Capsule, and not entirely understanding how the sequential, periodic backup works, I have some questions about how long I have before I reach the "dead end" of being told I can no longer store a complete CURRENT backup, and alos about what my options are.
To my uninformed mind, the roughly 600 GB of current data on the 2 machines SHOULD be albe to be contained in a 1 TB backup drive.  I do understand, however, that this backup strategy requires MUCH more space to be carried out as it is.  But I just don't understand exactly how this will play out.  Will Time Machine keep deleting "old" backups until it ONLY has a "current backup"?  It seems like there SHOULD be enough space for at least a little bit of past data for each machine it the total amount of data on the 2 HDs remains in the 600--700 GB range.
Obviously, my Time Capsule is living on borrowed time, and should be replaced with a nice, new 2 TB Time Capsule.  But I'd like to avoid that expenditure for a little while, if I can.  But I don't want to NOT be be backed up at least for my CURRENT data. Will that happen when my total data amount is still 2-300 GB less than the total size of my Time Capsule?
Are there any other intermim stategies?  Adding a second drive? (I believe I read Time Machine can't bridge multiple drives)  Is it possible to upgrade the drive in my 1 TB Time Capsule? (Probably is, but not sure I want to rely on a "home built" machine to backup my data). 
If and when I eventually get the larger Time Capsule, it would seem obvious that, since I'll have precious little in the way of "old backups", I should probably just start fresh and  not transfer any data over.  But is there then any use for the old Time Capsule?  Can you have 2 Time Capsules in your network?  I assume that you'd have to turn of the Wi-Fi router (of the one that isn't connected to your internet source I suppose?) since you're just using the old Time capsule as a network drive.  I assume that one would then chose one for each machine. 
Sorry this "question" is so long!  I'm thinking out loud a bit here.  But any insight about how the internal operations of this "black box" Time Capsule actually work, or any suggestions would be most appreciated. 

Answer 1

First of all to understand the magic of TM.. read this.
http://pondini.org/TM/Works.html
If you move around a lot of large files the space is used very quickly.. even worse is having TM still backup a virtual disk from parallels or bootcamp or even some cloud store.
Anything that piles large files on the disk and changes them in a way that requires TM to backup the whole thing.. exclude it.
So, not entirely understanding how Time Machine apportions space when two computers are using the same Time Capsule, and not entirely understanding how the sequential, periodic backup works, I have some questions about how long I have before I reach the "dead end" of being told I can no longer store a complete CURRENT backup, and alos about what my options are.
You might already be close.. TM needs a bit of free space to work with.
So you always want about 15% of the drive empty.
TM is like Apple.. winner takes all.. make sure the little guy gets nothing.
First come best dressed.
You can set sizes of the sparsebundles to make it a bit fairer.
See A8 here. http://pondini.org/TM/Troubleshooting.html
Obviously, my Time Capsule is living on borrowed time, and should be replaced with a nice, new 2 TB Time Capsule.  But I'd like to avoid that expenditure for a little while, if I can.  But I don't want to NOT be be backed up at least for my CURRENT data. Will that happen when my total data amount is still 2-300 GB less than the total size of my Time Capsule?
It should keep deleting old backups but you will see it slows down and takes much much longer to handle backups.
Are there any other intermim stategies?  Adding a second drive? (I believe I read Time Machine can't bridge multiple drives)  Is it possible to upgrade the drive in my 1 TB Time Capsule? (Probably is, but not sure I want to rely on a "home built" machine to backup my data). 
Simple one.. plug in a USB hard disk.. 2TB 3TB even 4TB.. It cannot bridge drives.. but you can start a new backup on the USB or partition the USB and use it for both.. so when you buy the new TC.. you just plug the USB over to the new one.
You can also replace the internal disk .. it is not hard actually.. a standard 3TB drive is a great boost.
But only on Gen3. An early gen1 or gen2 will have the power supply fail due to the new hard disk taking too high a current to spin up.
http://www.ifixit.com/Device/Apple_Time_Capsule
Lots of info around the web on doing hard drive upgrades.. I have done several . .no issues.
As long as you upgrade or figure out what the power supply is going to do.. !!
If and when I eventually get the larger Time Capsule, it would seem obvious that, since I'll have precious little in the way of "old backups", I should probably just start fresh and  not transfer any data over.  But is there then any use for the old Time Capsule?  Can you have 2 Time Capsules in your network?  I assume that you'd have to turn of the Wi-Fi router (of the one that isn't connected to your internet source I suppose?) since you're just using the old Time capsule as a network drive.  I assume that one would then chose one for each machine. 
Sorry this "question" is so long!  I'm thinking out loud a bit here.  But any insight about how the internal operations of this "black box" Time Capsule actually work, or any suggestions would be most appreciated. 
Always start fresh.. TM backups get messy after there has been loads of deletions.
Keep the old backups on the old TC for a while.. until you are sure nothing is likely to be lost. Then wipe the old one. As I said if it is Gen1 or Gen2 just sell it on ebay before it dies.
Gen3 can be worth keeping but you simply plug it in by ethernet to the New TC.. you can even leave the wireless on and setup roaming network.
But there are lots of variations.