Ooh, this can (and probably will) lead to a lot of headaches. I think this is overall a bad idea. Here's why:
If your network provisions you a new LAN address each time you connect, any active TCP connections will be terminated when the "handoff" occurs. The only way that your network could give you the same IP is if you set up a static IP, but then you would have to have two static IPs instead of one, and know which one to use at the right time by changing your routing table.
If you have any programs listening on the LAN IP of one card, they will stop being able to receive connections (or communicate through existing sockets) once the old LAN IP dies. In the worst case, if the daemon isn't listening on 0.0.0.0, you'll have to restart the daemon to listen on the new IP.
This sounds like it's going to be very unreliable, no matter how you try to hack it. You see, the wifi spec has explicit support for physical layer (and link layer) handoff from multiple AP nodes in the same network. So if you have 5 wifi APs in the same building that are configured correctly, you should be able to walk from one to the next without getting any connection interruptions. It should "just work".
If it doesn't just work, then either the APs don't overlap in their broadcast range enough, or they aren't configured to be part of the same network, so each one tries to act like a router and tries to give you a new LAN IP.
More info: http://www.wireless-nets.com/resources/tutorials/how_roaming_works.html
A long shot, but some information about how you MIGHT approach this (but I think you'd need a lot of help from the IT department maintaining the wifi network): http://en.wikipedia.org/wiki/Multihoming
My suggestion? Use cellular broadband. Wifi sucks. Really, it does. See http://www.tomshardware.com/picturestory/571-wi-fi-beamforming-networking.html