Hello fauj
I am only guessing here right now. But it should probably work if You check out as registered users. The guest user might have some data that could be shared between computers. There may be other reasons, but please make a quick test and see if it works doing it with two different users and in two ways. Firstly with two new users registrering at checkout, secondly with two already registered users.
regards
Jörgen @ Kreativ Fotografi