For some reason, it wanted to "force" (for lack of a better word) the dual monitor feature to use the highest resolution available for both, instead of allowing us to set the primary for one and the secondary for another.... Once we clicked through that a bit, we were able to adjust the resolution.
Thanks! (It seems the standard two pieces of advice are always: 1) Reboot, and b) Update your drivers. So after doing both of those, I was stuck - so thanks!)
rjb