Absent a catastrophic failure from tire chains or a wreck, usually you get a little warning. I have had cars and trucks come into the shop with corrosion pin-holes in the hard tubes, cracked and leaking hoses, loose bleed screws and other fittings and of course leaking wheel cylinders. The complaint has usually been, "pedal sinks slowly to the floor" or "its leaking brake fluid somewhere cause I have to refill it every so often;" or "the brakes pull this way or that or lock up suddenly."
These were complaints from naive drivers, not enthusiasts. My comment here is to say that if well maintained, a single bore MC is adequate especially on a doted-upon classic. That is not to say that a dual MC is not worthwhile, just that there are probably other things that take priority; like putting the factory equipment back in good shape: suspension, steering, original brakes, tires and so forth. They are the sort of thing that state safety inspections look for.
