The chances are that if one seal fails they will both fail. A single fail will increase pedal travel dramatically, and if you only had rears working with long travel, you may not have time to react before hitting the scenery.
I've had one total brake failure on a single circuit car, when on first application the pedal went to the floor. It happened on my drive, which is uphill to the road and at less than 5mph. It wasn't pleasant.
DaveW '05 Red Roadster S1 '16 Yellow (Not the only) Narrow AR GDI Plus 4
