Once again, your advice has all been very useful, thanks.
The 36 spokes on my XT / A719 touring front wheel were at about 100 kgf, and on the 40 spokes on the original tandem wheels (both front and rear) it was about 110 kgf. Although Mavic's web-site didn't provide the information, I found elsewhere on this forum that the A719 can take 110 kgf, so I took the touring wheel and evened up the tension a bit while increasing it to 110 kgf. I've now done a bit of stressing and fine-tuning and it looks like it will be good to go.
Switching the front rim onto the rear was obviously a much bigger task. I took it slowly and built up the tension steadily. It now appears to be well dished, round, and true, and the spoke tension is again around 110 kgf, some final stressing and re-truing and it should work well.
We'll take it out for a test ride this evening, and hopefully everything will be fine.