I'm assuming that one machine could actually connect to the internet via ethernet...
You don't specify the OS, so I'll assume XP and desktops.
just buy two wireless cards and setup the one that is connected to the internet via ethernet as an
ad hoc network.
Now before you get too excited about the $60 I just saved you (LOL) how far is 'opposite sides of the house'? if this is a fairly short distance it might be worth it to take your chances with those cheap cards that JamesXP posted.... however they don't list how many mw the cards are, so I'm guessing it's low... like 40-45mw or so.
It might be worth your money to buy a nice used wireless card like a cisco aironet 350 or 340. The 340's have less power but are cheaper. They both have external antennas that you could easily upgrade with a homebrew antenna for cheap if you needed any more distance. Or if you can find them I know some 340's and 350's have duel MMCX connectors so you could have multiple antennas if need be.
I think I might have a couple of these... I know I have a 350 pcmcia adapter... somewhere LOL! Let me know if you wanna go this route and I'll look around a bit.