Two Companies – One building ? Outa my depth…..

I am a small Newbie IT guy, whose company has purchased a secondary unrelated firm.

CompanyA.local.com is on a 90.0.0.0 segment
CompanyB.local.com is on a 10.0.0.0 segment.

Each only has one server (win2003). email and internet access is through a local provider.

They are both about to be moved into a new building and placed under one finance person.

What is the best way to keep both companies domains physically separate? But let my finance girl access all?

Would I run company a to a switch and company b to a switch and then both out to “something” and that “something” connects everybody to the dsl modem?

I am pretty lost here and really not sure what I need to do.

Thanks

Booster