Hi Nick,
On 707s in early 70s I can only remember 2 INS sets on the centre console. (Where was the third?) They were independent of each other, but we could load up to 9 new way points into both sets simultaneously using the "REMOTE" buttons. There was no automatic radio position updating.
The best we could do was press the "HOLD" button when we were over a VOR, compare the displayed position with the published, and update the position if it was in significantly in error. (In practice we hardly ever did it because determining the overhead position was always a bit iffy). All radio Nav aids had to be manually tuned. Navigation was done using Heading select, whilst comparing Cross Track Error and Desired Track, Dist to Go etc.
Later, with the introduction of FMS (on L1011) we had 3 INS, Tripple MIX, Radio position updating using DME/DME, ETA prediction etc. PFM.
Since INS was developed initially for Intercontinental Ballistic Missile Guidance and the Apollo Space programme, I don't think Mach 2 or less was a problem.
I've enjoyed reading this Concorde thread more than anything else that's been posted. Please tell how the Nav displays evolved during their service.
Regards, RRR