If there are 2, they are always in pairs right next to each other. Can't miss the second one.
If the one you changed was the STK 393-110, it is a 6 channel chip so there is only one of them in the set, which always requires 6 channels. Usually 2 chips 3 channels each, but that one has all 6 in it.
And yes the chips can fail without popping the fuses supplying them. Happens all the time.
Sounds like you did the right thing.