So it's clearer, these are the various different arrangements I think have been discussed so far, shown A to E all in one diagram:
I think we've, finally, reached agreement that A and B are different, and that, for the same individual piston area, distances between piston to hub centres, and hydraulic pressures, the two pistons in B give twice the brake effort of the single piston in A. That's because the force between the disc and the one pad in A is half then total force between the disc and the two pads in B, not because there's any more area of pad or number of pads in B. That two pistons pressing separately on the disc gives twice what one piston on its own gives is what I always thought was obvious and didn't understand why it was being argued against.
B is, of course, the two piston fixed calliper as per the Sprint, and C is the single piston sliding calliper, as per the Ford Sierra, etc., but normalized to the same sizes as the Sprint one. I think that there is and nearly always was agreement that B and C are equivalent and, for the same individual piston area, distances between piston to hub centres, and hydraulic pressures, give the same brake effort.
In case there's any doubt (please God no), C gives the same as B because the piston and cylinder both act to apply force to compress the pad-disc-pad sandwich and so that the one piston in C gives twice the force of the one piston in A, and thus the same as B.
D is what happens to B if one of the piston seizes and to C if the slider seizes. That is, only one piston is able to act on the fixed disc. I hope it's clear, it's the same as A, which gives half the brake force of B and C.
E is a peculiarity. It is the single piston fixed calliper sliding disc I introduced (spuriously it turns out) when trying to excuse why I got confused by the Sierra calliper. Though I did rapidly realized why C gives the same brake effort as B. I don't know that E exists anywhere outside this thread (but I hope not). My guess is, that if it does exist, the disc rattles about something horrible when not under braking.
I have to admit I was wrong in thinking that E was different from C. And I now realize they are the same. However, it was never intended as a suggestion that such an option as E could ever be a good idea for a brake system, any more than A, and frankly, I so wish I'd never mentioned it.
What may have been part of the confusion is what happens to B if one of the pistons seizes and to C if the slider seizes. Assuming the piston/slider seizes so it's pressing the pad that's no longer operated close up to the disc, and there's more end float in the hub bearing than needed to close the disc against it, then, initially at least, E works more or less like C. However, as the pad that's not operated wears away, the compliance in the bearing won't be enough to allow the disc to press on it, this setup will become like D, and give half the brake effort of B or C, like they do when they're part seized that way.