You should transfer the files you want the same way you would transfer any other files. Just put the music you buy on one computer onto a flash drive, for example, and then copy it into the itunes library of the other. The second computer will already be authorized to play it so it should play without any problems.
An even easier way is just to use file sharing over your local network. Just to be absolutely clear, there is no difference between the files a mac or a pc downloads from itunes, they will play on either just fine.