Unfortunately, LAVA is down right now; but there was an interesting discussion there.
You should be able to see most of the discussion by doing a google search on "Are XControls In or Out? - LAVA" and viewing the cached version...
Only if the XControl does not use ANY of the class VIs. Otherwise the XControl will lock all of the VIs of the class.
That would maen that if I had VI open with the XControl I could not edit my class. What a pain taht would be.
Why would I want to do that?
Well, the problem with two-way locking aside.....
It would allow easier debugging like being actually able to SEE what is passed into class VIs instead of a generic "lvclass" icon.
IIRC, it's a feature which is marked for possible future release (but not any time soon).