You may mean single TRS breaking out into dual TS phone plugs. Or are you trying to have multiple personalities listen to the monitor mix. TRS->dual TRS.

If it is the latter you should consider buying a headphone distribution box. The come in all versions of breakouts from two up to ....

Logic check:
You're using an adapter to make 2 headphone jacks available from 1?

If so, yes - the volume will go down.

Yes, Single TRS -> Dual TRS.......I just wanted to make sure a volume decrease was supposed to happen, which I thought would, but just wanted to know for sure.

http://www.sweetwater.com/store/detail/HeadAmp4/

This will fix your problem.