I've sorted this out, and am posting for the benefit of anybody else who might benefit.
The Multirc plugin script puts values of 10000, 20000, 40000 and 80000 in /proc/stb/ir/rc/mask2. For me, this doesn't work.
You CAN control two 8000HD boxes individually without the need to program with a special piece of hardware, as was suggested in a post which seems to have been removed.
What you need to do is to put either 1,2,4 or 8 in /proc/stb/ir/rc/mask2 and 0 in both mask0 and mask1.
1 corresponds to both DIP switches off, 2 to 1 on, 4 to 2 on and 8 to both on. That works for me.
I have a startup script to activate the settings (as I have always had for the other remote types).
Why DMM couldn't have just said this, I don't know.
Anyway, all sorted!