For anyone reading this in the future, I eventually gave up on getting this to work.
Firstly, I was having trouble getting my “Combined Playing Technique” Add-on Switches to correctly activate in my Expression Map. Even though this worked in my dummy example posted above, it wouldn’t in my larger, more-complicated, Expression Map (with lots of other Add-on Switches). I found a post from Daniel (Add-on Switch Weirdness - #2 by dspreadbury) suggesting that “Add-on Switches that define a combination of two or more playback techniques are not evaluated”, at least in Dorico 4.2, so that may have been causing problems.
Secondly, I realised that it was somewhat cleaner to simply only set up 16 custom Playback Techniques to activate the 16 switches mentioned above (A0+B0, A0+B1, etc), and for each of these switches send 3 CC messages (one for the A value, one for the B value, and one for the combined value). This did mean a lot of switches, and some redundancy of information, but it worked reasonably well.
Thirdly, I found another post from Daniel (Expression Maps: How are switches selected? - #4 by dspreadbury) containing a useful description of how Expression Maps decide which switches to use. This made it clear to me that my request wouldn’t work with the current algorithm, which looks for the “best” Add-on Switches rather than necessarily activating all valid Add-on Switches. The corollary of this is that Add-on Switches need to be distinct to guarantee selection.
Finally, I discovered that I didn’t actually need these “combined switches” anyway, so swiftly moved on. 