Hello,
First question:
You can perform a DME updating procedure by the CDU linked to AP.
The problem is that you cannot change  VOR/DME and when the leg is changed, maybe you cannot follow any more the plane navigation.
This is the why it's better to use  a DME on track with the CDU of the INS linked to AP  and use  other CDU with a DME off track.
Second question:
when,during long legs without DME on track where it's not required to follow continous HDG changing you can select two DMEs off track
and perform a DUAL DME updating.
Cheers Gianfranco

HI,
the DUAL DME updating doesn't create any problem.
But to follow the navigation you have two possibilities:
Tune on VHF NAV1 a VOR/DME on track and on VHF NAV2 a DME off track   or tune on VHF NAV1 a DME off track and on VHF NAV2 a VOR/DME on track.
You can also tune both VHF nav on two DMEs off track and link the AP to your desired INS but in this case you loose the possibility to follow the navigation and check what the INS in command is doing.
Gianfranco