Ah, I've just noticed that each individual target can have its own zone. So I just need to create 2 identical sets of 16, job done!
It's a bit odd that the workspace comes with target cues, but they are set up for 8 target points only, so you have to mess about a bit just to get them to work nicely.
One more question: is it possible to fade the target beams on and off rather than just switching on and off?