I wanted to try dayz out so I used my friend's steam account to install arma 2 and arma oa, then installed dayz and the launcher. I tried it out for a few days then decided, ok I want to buy this so we can play together. I only had to buy my own copy of arma 2 and arma oa on steam and that was it, I was playing on my own copy and had my own character in dayz. Nothing else to it. My friend came over and we both played dayz on the same ip as well, on two steam steam accounts with two copies of arma 2. On a side note, you can copy paste arma 2 and oa to your own steam folder saving you from having to redownload. Just make sure you "verify integirty of game cache" once you've done that by going to: games > games library > right click arma 2 + arma oa > properties > local files > verify integrity of game cache... You may not have to do that but I needed to for it to work as it was missing 1 file on both.