They can both use the wireless network at the same time without any problems, but you didn’t mention if the groups share files or other resources apart from the internet.
If they do, you could set up the two groups to be in two separate workgroups to share any resources between themselves, but still keeping them separate from each other.
I guess they are just splitting the cost straight in half as opposed to having it based on each others actual usage?
Setup a server to handle the dialing (basic PC with a modem will be fine). Setup internet connection sharing on the ISP Connection with demad dialing enabled. Connect the network card on that machine to the a network switch (not a router). Connect the uplink port (also called the WAN port) of both wireless routers to the switch. You now will have two wireless networks setup sharing an internet connection with no access between the networks.