I understand now Mark.

I did try the duel INS in the 727 at first and found the updating to be quite fast when using on-track and off-track together. Then I tried just the on-track updating and was okay with the slightly longer update time. If you use enough VOR/DMEs in your flight plan the accuracy will stay at '1' when just using on-track updating. Also a duel set-up takes up too much screen space as I leave the CDU showing most of my flight. I could describe how I update my single unit if that would be helpful.
I also look at it this way: I'm in this hobby as a learning experience and to have fun. I understand the operations and principles of the INS unit. I really don't want to over burden myself by looking up off-track VORs online to find out the elevation info and manually enter that data into the second INS unit. No lives will be lost if I mess-up! (I hope)  :-/

Paul