UPDATE: YES, I can!
I tried Genoil first but couldn´t find anything on this topic, so switched to Claymore (10), no problem what so ever!
Just start mining, then type a number to disable/enable that specific GPU in runtime, for me this is "0" as I want to game with that GPU (just type a number, not the "").
I guess if I relaunch the miner it will enable all GPUs, unless you use commandline -di.
-di GPU indexes, default is all available GPUs. For example, if you have four GPUs “-di 02” will enable only first and third GPUs (#0 and #2). You can also turn on/off cards in runtime with “0”…”9″ keys and check current statistics with “s” key.
I will say however; I have 2 cards in SLI, so in order to game I have to go into Nvidia control panel and disable SLI, to do that it requires me to exit apps - among others the miner - to change that config. And the same if I want to re-enable SLI for gaming.
So I guess I might remove the SLI bridge/leave SLI disabled and game with just the main GPU as long as it has enough performance.
SLI gives FPS boost in some games but not all, anyways (sadly).