Win 7 network with 2 separate businesses

Two small businesses share a common internet. both see each other's computers because both businesses share folders among themselves. Both businesses need to be hidden from each other. WIN 7 Pro Workgroups, homegroups don't help, blocking IPs doesn't work. I have second router, E1200, and need some clues as to it's setup to accomplish the separation. Win 7 Pro supports 4 of the six computers which I am responsible for, the other two computers are Win 7 but not Pro. Hardware items are WRT54G2 router, and two Cisco switches to connect the 7 computers and 3 printers.