Realistically, what you want can be done, but it's trivial to circumvent
should anyone want to.
Your question about "how can the form differentiate between the two in the
same time?" makes me suspect that you haven't split your application into a
front-end (containing the queries, forms, reports, macros and modules),
linked to a back-end (containing the tables and relationships). Only the
back-end should be on the server: each user should have his/her own copy of
the front-end, ideally on his/her hard drive.
(Even if you haven't done this recommended step, Access has no problem
distinguishing between the users. This is because a copy of the form is sent
to each workstation: one of the reasons to split, to reduce network traffic)
Doug Steele, Microsoft Access MVP
(no e-mails, please!)