I too have been struggling with exactly the same issue. two monitors on host, one on client, no easy way to swtich back and forth without mousing up to the menu bar. i should mention that i'm using 10.8 but as i didn't find this thread in the 10.8 forum just thought i would chime in here.
i've come up with one workaround that is a slight improvement for me:
Option-Command-X (switch to observe mode) then immediately
Option-Command-1 (or 2, to switch to the other monitor) then
Option-Command-X again to switch back to observe mode
I know it seems a bit much, but it's actually pretty fast once you get used to it and since you're already holding down Option-Command it's really only three keystrokes. for me this is a bit faster than mousing up to the menubar. hope this helps someone.
On interesting thing (BUG?) i've found is that while in observe mode i am not able to quickly switch back and forth between monitors as expected using Option-Command-1 and Option-Command-2 in succession. For some reason I need to switch back to control mode and then back to observe mode before it will let me change monitors using key commands. another way it works is if i just click somewhere on the menu bar first which defeats the purpose of using key commands. pretty sure this is a bug.