If I understand you correctly - and I'm not sure that I do to be honest - it sounds like you're misunderstanding/misusing ther term "mono".
"Mono" means all program material is funneled to a single sound channel. Seperate L and R channels are two halves of a stereo signal and not two "mono L" and "mono R" channels.
As far as your situation, a lot depends upon how you have your Control Room channels configured, but assuming a pretty standard configuration, when you click the mono button and select a loudspeaker, yes you should be getting both channels in that loudspeaker. If you instead just want to hear the the left channel out of the left spekaer, for example,, you should be able to solo select that speaker without hitting the mono button. This, I believe, should solo just the left channel.
G.