Hi, Rick.

I think the special setup on the 707 (INS/DOPPLER switch and dedicated NAV mode for the AP) binds the CDU#1 steering input to the plane, no matter what. Even regardless of the nav switches on the INS popup. So I guess your plans on using the #2 unit to steer the AP will fail. At least, I wouldn't know how to force a CDU#2 priority on this plane.

For all other purposes like cross-checking or just some second display reference, the #2 unit will do fine.