The crack in the head could have been the cause of the overheating or the result of it. If it cracked and you lost coolant, that could result in overheating, or like you said, if it overheated (badly) for another reason, a cracked head might result.
Either way, that's a problem that needs to be dealt with. Should you replace both heads? Hard to say. You could consider putting some epoxy on the known leak to temporarily seal it up long enough to run a pressure test. If the system holds pressure you could take your chances and just fix that one side. Remember, you might have a leak somewhere else and that other leak could have caused the coolant loss which resulted in the overheating, and the cracked head could have resulted from that.
The problem is, it's a big teardown to pull one head, and not much labor increment to keep going and do the other side at the same time.
Tough call.
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