Well, of course he can. But if you read his original query carefully, you might see it expresses the need to keep totally separate accounts, to have it simple. Obviously, he was already trying to sync both devices to the same library and having problems with it. Obviously, he could have iTunes installed on another computer entirely... but him being here to ask the question indicated that he'd rather not do that...
But I stand corrected, switching library is such a headrush... whoaaaa! Oh, but wait, perhaps I don't want to have a common music library with my girlfriend, and have our apps all together. And by the way, we do use the same Apple ID, which allows us to sync pictures together with photostream, and to choose to install apps the other downloaded. What I like though is that we can use that and at the same time keep completely separate use and sync of our devices : her apps don't get into my library unless I force the transfer of my purchases, which I just don't need to do as all MY things are always there. I don't have to check what to sync and what not, I just sync everything, and backup everything, when needed, as needed, on MY library. Also have an iPad for the whole family, and it's also on a separate library with a bunch of apps for my daughter, and I like it that way. I know there's a way to make it work on the same library, I did that before. And then I learned about the separate libraries option and I never looked back. To each his own, it seemed pappy was ok with my idea...
It was ok for you to remind him it was possible on the same library, I understand that. Just suggest it, don't quote me as if I'd suggested the most terrible solution there could ever be for someone who obviously wanted to have completely different accounts on the same iTunes...