Will the two interfere with each other?
Impossible to say unless you try.
Unless you really need two separate wireless networks, it would be a good idea to turn one off to minimize the chances of wireless interference.
If you do need two networks, keep the routers separated by at least 5-6 feet or a couple of meters.
To add to Bob Timmons' post, it is possible to turn some ISP modem-routers into "modem" or "bridge" mode, which means that they act purely as a modem, making your Extreme the router (the Superhub springs to mind).
Retrieving data ...