My apologies to all, I am repeating some of what has already been posted.
In my experience, all speedlights configured within a given remote group will fire at the power level/exposure compensation specified in the Commander Mode using the popup speedlight, or in an on-camera Master SB700/800/900/910 speedlight.
If you want different power level/exposure compensation for each of the two remote SB600s, they must be in different groups.
In Manual Mode, you can specify the exact power level desired for each remote group: M1/1, M1/2, M1/4 etc. All remote speedlights in each group will fire at this level.
In TTL Mode, you can specify the exposure compensation desired for each remote group + 3.0 to - 3.0. All remote speedlights in each group will fire at this exposure compensation level.
Nikon suggests no more than three speedlights in any remote group. I have successfully conducted night shoots with several more than that, but it takes careful placement of the remote speedlights relative to the on-camera Master/Commander.
As George mentioned, the Infrared (IR) CLS preflash signalling does not necessarily require line of sight alignment between the remote speedlights and the on-camera Master/Commander. In small to medium size rooms, the longer wavelength IR signals will bounce off floors, walls, and ceilings and find their way to the remotes. In very large rooms and outdoors, or outdoors, line of sight is more critical. I have had remote speedlights in large umbrellas fire around corners in separate rooms when doing interior photography. As George also mentioned, you will have to experiment.
Configuring remote speedlights in Commander Mode is a bit tedious. It gets easier when working with an on-camera speedlight in Master Mode.
Hope this helps a bit.
HBB in Phoenix, Arizona Nikonian Team Member
Photography is a journey with no conceivable destination.