3 Computers On 2 Different Wireless Networks?

One of my business customers need a network set up a certain way.They have two XP computers right now. Their internet connection is wireless through a Verizon Mifi. They also have a crossover cable through the wall so the two computers can share files.They are going to add a 3rd Windows 7 computer and turn one of those XP computers into a server where all the files they need to access (Quickbooks, etc) will be stored. They do not want the server to have internet access at all. Becuase of the way the building is set up, I need to add a wireless router to the mix. The Mifi will only be for the two client computers to connect to the internet and the other wireless router would be for the two client computers to connect to the server to access their files. I tried yesterday to add the wireless router to the mix by having one computer (XP) use one wireless card to access the Mifi with an IP address of 192.168.2.1 and another wireless card to access the wirless router with an IP address of 192.168.1.1. It connected to both perfectly, but then got confused as to which wireless connection to use to access the internet. I've read about how you need to use two different gateway addresses on each to seperate them inside Windows but wanted to make sure that's how I needed to do it before I went out there and tried it.