.... actually the two callipers connected to one brake master cylinder may not be as problematic as I first thought.
as mentioned earlier, I am running mismatched brake and calliper (simano XT and magura Marta) on the rear and on the weekend I bled that brake.
The amount of lever travel is the same or less than for the front whwich is full Shimano set up.
So given it some thought, what I think happens is when the lever goes back to its resting position by means of the internal spring, the pads are pulled back by the brake fluid. If there is a mismatch, i.e. the calliper piston diameter is not as per manufacturer's, then the pistons and subsequently the pads will be draw out more or drawn out less than intended.
If they are drawn out less (pistons are larger diameter than intended), providing there is still clearance between pad and disc, there is no problem and lever travel is the same for braking.
This will still be the case running two callipers off one master, but the clearance between the pads and disc will be less but as long the pads do not rub, all will be well.