OS X V10.7 Lion :: Two Separate Time Machine Backups / Use To Create Two Separate Profiles On One New Computer

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 :-)

Info:
iMac (27-inch Late 2009), Mac OS X (10.7.3)