My dual MC failed in the driveway so it was OK. There is a long steep hill leading to a coffee shop we regularly visit. If the MC had failed on that hill, we could have had a serious accident.
When it failed there was no warning - pedal went to the floor - no brakes - front or rear. After the new MC was installed the brake warning light was one since the rear brakes were bled first. It went out after a couple of hard stops.
A long time ago, we had the rear brake line rust out. In that case, the dual MC worked as expected. The brake warning light came on but the truck could still be driven until the line was replaced.
Dual MCs would provide safety, but I think they are only found in race cars.
