In reply to it624:
Have you already got axes from Scottish winter? If so, which one(s)?
If you have a pair of technical or semi-technical axes, I'd throw them in the bag and then if in doubt take a walking axe with a technical axe as back up.
I'd echo the view that it really does depend on the route.
In my somewhat limited experience one mountaineering axe (I use a DMM Raptor) is good for most stuff up to AD, but not all. E.g. the more moderate face climbs on the Tacul (Contamine Grisolle etc.) are definitely two axes for most people. And depending on the conditions even single axe routes can get interesting if steep snow slopes are icy - e.g. on the Rochefort last year there is an abseil down a 50-55 degree slope in the middle of the route (you come back the same way), and when we did it it was hard ice. A fixed rope had been placed to aid return but it was frozen into the ice from part way down. We had a single axe each.
My friend had the forethought to leave a bunch of our slings and draws to act as substitute fixed gear up the steeper section on our return, and we were glad for it - we'd have got up it with single axes, I'm sure (or could have maybe let the leader go up with both), but without the fixed gear a second axe would have been desirable at least for the leader.
I also heard that a lot of people were glad of two axes on the approach to the Forbes last year (and the descent!), although I didn't do it myself.
Conversely stuff like Cosmiques and the Entreves, Marbrees traverses should be fine with a single axe.
(I know most of the routes I've mentioned are AD rather than AD- but I'm not aware of that many AD- routes around Cham off the top of my head and I can imagine the desire to get on some of the AD classics may be there if you're going well.)