You're going to need to get one additional router.
Run one network directly off the wireless router you already have and run the other off the new one. Go into each router and set up separate networks. This will separate the networks (not entirely, but enough).
For example make one 192.168.100.0/255.255.255.0 and the other 192.168.200.0/255.255.255.0
If you need complete isolation, buy two routers instead of 1 and run them both off the wireless router. Make the wireless router use the 192.168.0.0/255.255.255.0 network. Have the other two use the networks I have above,