From OperationsManual.pdf page 13 -

3 DME POSITION UPDATING

The INS positions may be updated by a single or dual DME (Distance Measuring
Equipment). Optimum single updating may be obtained selecting a DME station “off
track”, at least 15 NM off the intended track, and within 200NM of range. Optimum dual
DME updating is obtained tuning a second DME along track. INS 3 does not require DME
data insertion, since the INS 3 receives DME updating from INS1 and/or INS2. Tuning
both VHF NAV on the same frequency will not improve the single DME updating both in
efficiency and velocity.

This tells me it CAN be done. So I follow the instructions that follow this, but I really think that where it says on page 14:
Tune on VHF-NAV panels (n°1 and/or n°2) one DME or two DME stations, for a single or
dual DME updating as desired. INS 1 is connected to NAV 1 and INS 2 to NAV 2.
is where I become unglued when attempting to update. I seem to get a lot of trouble finding the DME station to enter, so I never seem to get it updated.

I know CoolP provided some great info in the CIVA INS - THE ONLY THREAD PLEASE (starting at reply #149 through to reply #158) on how it is done, but I still wasn't able to do it. I tried to achieve this twice, but I'm not sure I ever tried it again after those failures. I felt it best to do the same route, SEA - HNL (KSEA - PHNL), where I had all the data written down, so as to achieve a result. I'm pretty sure I also tried it on a flight from MEL - NAN (YMML - NFFN), but I failed again.

What is really strange here is that I have picked up something CoolP put in his reply (#154) that I must have completely overlooked or misunderstood, that now makes sense so I will try again. It has to do with the plan converter and the ADC file for the flight plan. I somehow thought they went into the plan and I somehow had to take the waypoints out of it so I wasn't doing some zigzagging in the flight. It's amazing when you read something, after you haven't read for a while, and it now makes perfect sense! I'm off to try SEA - HNL with this newly discovered (for me) info!

So here's hoping it works. ;)

Mark