Joining and logging to another domain

My network is Win2k, say that I have 2 domains (A and B)with different forests, but join the same wiring media, so from my pc I can reach both.
I joined domain “A”, when I tried to join the other domain “B”, I found that I disjoined from domain “A”. Is there a way I can join both, so when I can log off from domain “A” and log to domain “B” without the need for disjoining from a domain and join the other every time? And also without creating trusts between the domains?