I'm still interested in a way to do this within attractmode, but for now will be using Actual Multiple Monitors (at least until the trial runs out).
This will send Attract mode to monitor 2, but needs exceptions set up for MAME, model 2, Supermodel and WinUAE - this way Mame can open where i want, as will WinUAE, and Model 2 and Supermodel will go fullscreen automatically on Monitor 1.
Essentially i needed Model 2 to open on monitor 1 because AMM couldn't force it into fullscreen on monitor 2. It can do that with Supermodel, but...
Forcing something to another monitor won't change res in the emulator, of course - it just scales the screen to whatever res that monitor is on. So If i had Supermodel running at 320x240 (or whatever resolution it opens at on monitor 1, and forced it onto monitor 2 that was set to 640x480, it would scale to the new res and look really bad while at the same time still flickering. I'm not sure how accurate any 480i to 480i transition is, whether perfectly aligned or not, but Attract Mode can take one for the team - and anyway still looks fine when doing this as both screens are set to 480i.
Now to dive into Demul