Having a major problem with my sons ipod, 2nd generation

He has downloaded a few applications on his mums computer, bought properly from the itunes store.

THen, he comes over to my place, and we install itunes on my computer, and buy 4 new apps from the itunes store.

He didnt know his username or password, so we just made up another profile. Was this a mistake? Do you have to just have one itunes account? Is it possible for me to merge my apps on these 2 profiles?

When we synced the ipod, the apps on his phone were getting deleted. So, we cancelled the sync when the third app was deleted.

WHY does this happen?

Surely there is some way for people to use multiple computers and multiple sign in accounts?

All help much appreciated

It is possible, my daughter uses her Touch with apps purchased from 2 accounts. I think you need to log into the other iTunes account and then authorize your computer for that account so that your computer is authorized for both accounts.