I've done something similar before (swapping wifi to ethernet, and having both up at the same time, but on the same network), but it would invariably disconnect whatever connections I had established over en1 and re-reoute everything to ethernet (breaking iChat videoconferences, re-establishing ssh connections, etc).
But if you manually configure your routing table, I'm sure you can use both simultaneously for different destinations (i.e., tell your mac to go to google via airport but yahoo via ethernet). The procedure for this is surely to be found via google.