I suspect your thinking re dual circuit brakes would in past times have most folk still running their cars, but in these litigous times there seems little chance that folk can evaluate risks to any degree as wee might have in past times..? Dual circuit brakes tend to start off with a master cylinder supply tank that is split into two seperate sections each section feeding it`s half of the braking system via seperate inputs to the master cylinder. The master cylinder is usually operated by one pushrod acting on both circuits. the pistons are linked one behind the other each with it`s own seal, thus two seals acting on the cylinder. With the system split in this way enven if one system fails to the extent that all the fluid leaks out the other system should still operate with it`s seperate fluid supply.
If there is an internal failing in the master cylinder as in the brake pedal might feel softer than usual, if possible take the lid of the master cylinder and while looking in then have someone apply the brakes hard and check to see if there is any disturbance on the surface of the brake fluid.... Usually if a seal is failing tsome of the he pressure applied to the master cylinder can escape back past a failing seal to cause disturbance as described...
Hope this helps and does not muddy the water... I am NO expert.
