I'm still looking at the possibility of a two-horse BIF, looks like it would need a 768x768, and there's some number-crunching to do for the scaling, but I'll be trying it out in due course (so long as the numbers work). The one worry is whether it will have weird interactions with riders - and I have a horrible feeling it will. But suck it and see....
Well, I sucked, and so did the results

I tried the basic idea by converting the ArmHorse BIF to a 640x640 hi-res plate, and did the necessary rescaling of the rectangles to pick up the actions correctly. It sort of worked.... I got horses, and riders sat on them (well, no they didn't - they hovered over them).

Even with resetting the horse 'origin' coordinates to stop the hovering it still was not right. What should have been mighty, armoured warhorses ended up as donkeys wrapped in tinfoil. The problem is that mounts don't have a 'scale' entry in the unit_prod file, and will just come out relative to their BIF proportions - ie at four-fifths of the desired size.

In the long run it might be possible to get it to work by also re-BIFing all the potential riders, but it soon becomes a major project, and I'm not 100% sure it would work out even then, as there are too many factors working in opposite directions. So for now we'll have to be content with one mount per BIF.

On the plus side, combining the euthitonon and oxivolos BIFs has worked