It's not so much a matter of pistons quantity, but rather asymmetrical load on fork that twists it out of line.
So I suppose a bike with a chain drive has "asymetrical load" that twists a swingarm and really it
should have chain drive on both sides of the wheel to keep it from twisting off in the wrong direction?
The whole wheel gets slowed, not just one side. Drag load from the tire is transferred up the wheel
uniformly to the axle. Twist, if any would be up and down the fork with the brake disk trying to pull
the caliper around. But if the axle is not loose then it is hard to near impossible for it to pull the
other fork enough to rotate it around the caliper side fork.
Well built bikes are designed to be robust enough to counter that. Some that are not would need fork
braces to help keep them from bending at the top of the forks. When they do, it is more of a bending
that would tilt the wheel/tire toward the caliper side at the top of the forks rather than a twisting the
wheel to the left or right.
Dual disks are a gimmick mostly IMO. Only adding more complexity than really necessary when just
a single rotor big enough and with adequate brake pad surface and pistons large enough to apply the
pressure would do just fine thank you. Dual disks would need more hydraulic lines, brake pads, rotors
calipers and so more parts to maintain/repair/replace, more unsprung weight... And bleeding them
would be oh so much fun too.... NOT
But, they would probably tend to run cooler and there would be none of that dreaded fork twist going
on. Probably good for the racing bikes but not worth it enough for all the other disadvantages in my
view for bikes that are not going to be ridden at break neck speed most of the time.