There is a way to have two separate copies on the same machine, but you would need to mess with the Windows registry. I'd advise against doing this if you've never modified the registry before (as you can kill your Windows installation if you screw up), but it does work. Process : Open the registry (regedit.exe) and search for "Bohemia Interactive Studio". It should have a few subfolders in it, like "ArmA 2", "ArmA 2 OA", and "ArmA 2 REINFORCEMENTS". Export the entire "Bohemia Interactive Studio" folder to a file on your desktop called, say, "ARMA2_FirstAccount.reg". Now delete the "Bohemia Interactive Studio" registry folder. Close regedit. Install and activate ARMA 2 & OA on your second Steam account. Run it at least once. The new CD keys will be written to the registry. Open regedit again and export "Bohemia Interactive Studio" with the new keys to a file called "ARMA2_SecondAccount.reg". Now, when you want to play on the first account, double click "ARMA2_FirstAccount.reg" and add it to your registry, then run the game. Same process for the second account and "ARMA2_SecondAccount.reg". A bit complicated if you haven't done something like this before, so it's up to you if you want to attempt it. And just a note that you NEVER want to share these .reg files with anyone else, because they contain your CD keys.