Unless both changed after the first failed, whatever condition caused the first to fail was present in both calipers – the second was just slower to fail. It is likely that the caliper pistons are not fully retracted or are partially caught on the slide pins, thus holding the brakes slightly. They should have replaced both sides, or at least disassembled and cleaned the left side at the same time.
Holding the brakes is something you should be able to feel. You can certainly try it – coast up to about 5 mph in Drive on a flat surface, shift the shifter to neutral and see how far you go. If the car stops quickly enough, your brakes are hanging up somewhere, assuming the tires aren’t overinflated.
