Resolution for a dual head fullscreen setup

on my Nvidia 8600mGT i use dualhead/spanmode
and i have to set max (native?) resolution
on the first monitor 1680x1050(vvvv app and Live)
and also on the second monitor (fullscreen renderer @1024x768 which is the max resolution supported by the monitor)
if not set to max on both screens …
live graphic interface freeze …still working but
with an hole in it :) back to normal …when disabling fullscreen mode…
is it normal about that max resolution ?