There's a tutorial doc in the Civa folder. Step 7 - DUAL DME UPDATING speaks about the dual DME operation, but I think we've described the procedure more or less in this thread already. It starts here. http://www.captainsim.org/yabb2/YaBB.pl?num=1285100008/138#138
A shorter list is here, plus some discussion. http://www.captainsim.org/yabb2/YaBB.pl?num=1285100008/156#156

I think I have way too long explanations there, but the Operations Manual pdf of the Cive gives you a step by step guide.
3 DME  POSITION  UPDATING is the chapter you are looking for I guess. The main item to keep in mind is the need to right click the numbers 7 and 9 while the selector is in DSRTK/STS. This will enter the submode of the DME updating. Leave the elevation data procedure out, that's optional stuff.

The single DME update isn't any different from the dual one, except for the operation of two units of course and choosing different stations for them. If you have one on your planned flight path and one with an offset, you will receive the best accuracy.

Most VORs in FSX range up to the maximum of approx 200nm, so you can tune the one on your flight path being 200nm out to overfly and, again, 200nm out. Gives you 400nm (minus the cone of silence) of usable data thanks to the (unrealistic) sim's accuracy on receiving stations. That's a comfortable range for the subsonic planes I think. That Concorde lady eats those 400nm a bit faster though, same as the fuel.  ;D