How do I just backup just one user account on Time Machine?

Hello everyone,

I've moved to a new computer with Mountain Lion and didn't have my backup disc available when I first set up the computer. Now, after restoring from my backup, I have two user accounts that I am trying to combine as per instructions here. I know it's challenging and I may have to reinstall the OS and start from scratch, so I'd like to backup before I do it. Unfortunately I haven't plugged in Time Machine since I've used Migration Assistant, because I didn't know if I could or if it'd overwrite my old data.

How can I back up the new computer without creating two user accounts (the ones I'm trying to combine) on my external drive? And please don't recommend just buying another external hard drive. Any advice is appreciated.

Thanks.