I want to use two separate Lion Time Machine backups and use them to create two separate profiles on one new computer

Question

This is something I need to do temporarily but I must do it. 
I have an iMac 27 (end of year 2009) I5.  It has 1 TB internal and I want to replace it with a 2 TB internal.
I have time machine backups on an external hard drive that has been connected via Firewire. 
I have another iMac (my son's)Imac 24 Intel Core 2 Duo that has it's own separate Time machine backup to it's own separate firewire connected external drive.  Both machines are completely up to date with the latest version of Lion and all other necessary software updates.
When the internal hardrive is replaced - I want to use my latest time machine backup to create one profile on the imac and then use my son's latest time machine back up for a second profile.  The end goal being to have everything from my current set up ie: apps, documents, itunes, etc existing in one profile and everything in my son's current set up existing in the other profile.   We can both be administrators for the time being as well.  I should note even though we both have some of the same apps, we both also have other apps that the other does not.
My profile will be using about 900 gigs of total space.
My son's profile will be using about 250 gigs of total space.
This is temporary until the new iMac is released.  Then I will want to transfer my profile to that leaving my son's intact on the iMac 27 I5.
I realize that during the temporary period that Time Machine will be making backups of this entire two profile setup, which I am now understanding as I write this will create a new issue for me when I want to transfer only my profile to the new computer as well.  But one bridge to cross at a time, I guess.
I need to do this because we must bring his current computer to my father - as his iMac G5 is inoperable.  We have been doing this round robin hand me down in our family since IMac DV special edition :-)
Any insight on this would be greatly appreciated.  And any other suggestions as well.
Thanks very much.

Answer 1

Hey Shootist000,  Thanks for the replies.  First off, I am actually okay if we both have each other applications so I am not concerned about that. It is still unclear from what you have said as to whether I can in fact, use these two separte time machine backup to create two separate profiles on the new hard drive.  If that in fact can be done, then - will it create two copies of the apps we both have in the ROOT of the drive as well as placing all the ones we each have separately?
And reading your second reply, makes me think that maybe the partition is the way to go to begin with and then down the road - after I am in the new iMac, I could reformat the iMac 27 with only one partion and restore my son's latest back prior to the reformat thus giving him full access to the 2 TB of the drive.
If this is still making sense, does this mean :
replace the 1TB with a 2TB - but have it partitione - 1.5 TB & 500 GB.
Install(Restore using Time Machine) my latest back up prior to the  1.5 partition
Install (Restore using Time Machine) my son's latest back up prior to the 500 gig partition
Lastly, in order for us both to be using the partioned computer - I am assuming Switch user can no longer be used - so would we need to restart and select our respective partions in order to use the computer?   And if so, how would we each be able to be using time machine to back up - two separte external hard drives - with each one dedicated to only one partition? Or ? 
Thanks so much.
Ironically, I am only trying to do this so that I don't need to manually install all his apps, setting, games, blah blah for this temporary period.  :-)