It's not that the master'll fail if you use a BA master with twin disks... it's more that the brakes might fail to operate correctly. The Speedy master has a wider piston which moves more fluid to operate the two calipers. Using the BA master with its narrower piston on dual calipers will have the lever bottoming out before the calipers operate fully. It'll probably be fine with new pads on a cold bike, but warm things up a little with worn pads and you could run out of brake real quick.
The opposite happens if you run a speedy cylinder with a single calliper - it'll get "grabby" and brake way too quickly.
If it wasn't important to have the different sizes, do you really think Triumph would have done it at extra cost?
