I have several games on D2D and EA. I have two different PC's I use (one I use in my PC room and the other I hook up to my home theater). I have a few of the same games installed on both machines with no problem (Need4Speed shift, Battlefield Bad Company 2).
I haven't installed the new games I just got this week from D2D yet (Need4Speed Shift 2, Mass Effect 2, or BulletStorm), but in the past, you just download a big install file for the game. I would store this file on a back up drive and could use the installation on another PC. I have all of these huge folders for these games saved on a USB harddrive, so I don't ever have to download them again. Just install from the folder when I need to. The games that have a multiplayer mode, like Bad Company 2 and NFS Shift, you can only use one copy at a time. Its just like Steam in that sense. The multiplayer login is locked to the CD key of that particular game.