I assume both heads were tested the first time and only 1 was cracked.
Where was the crack?
between valves
Did you hear any "pinging/knocking"?
pre-detonation causes cylinder to heat up above normal which doesn't show up on the temp gauge right away, and aluminum heads crack easier than cast-iron with heat.
Did you notice excessive carbon build up on pistons when the first head was replaced?
There is a very simple "balloon test" you can do to see which cylinder might be leaking, before pulling everything apart.
Balloon test:
With cooling system intact(all hose connected and tight), with or without coolant, remove all spark plugs.
Remove rad cap and overflow tube
Plug overflow outlet tube, I often use a vacuum cap off the engine.
Place a balloon over the rad cap opening, I use a latex glove and rubber band.
Put one spark plug back in, I use my compression tester instead so I can do a compression test at the same time.
Some suggest pulling the EFI(fuel injection) fuse at this time, I don't, so up to you.
Turn engine over a few times, watch balloon.
Remove spark plug and put it in the next cylinder to test, repeat for all cylinders.
Balloon with dance around if/when you get to a cylinder that has a blown head gasket or cracked head, can't miss it.
I check all cylinders even after I find one that makes the balloon dance...you never know there could be two.
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