Two Workgroups For A Laptop?

I take my laptop back and forth to work, and have mixed XP/W7 networks at both places. (the laptop is W7 Prof 64bit). Obviously, the workgroup names for the networks are different. (No homegroup for home because this is the only W7 machine on that network).In both cases, I can see some of the other computers, but not all, and they can't access me, even though we all have file sharing on.Do I need to have two different workgroups on my laptop? If so, how do I set them up? I can access both networks both wired & wireless, if that matters.