Two networks, or two internet connections?
If just two LANs, your computer should choose the network depending on the address you enter.
If you have two internet connections, it is generally OS/hardware dependant. There are a couple of routers and whatnot that will let you use two different internet connections, but I don't know how much control you have over things. They're generally more expensive than the usual SOHO crap people buy. Also, if your OS supports multipath or something similar, it should work. But again, I don't know how much control you get over things.