No one is saying it is or isn't cracked but the basic idea here is that there is likely a cooling source/passage that somehow is common between both heads. The most obvious culprits are the intake or the water pump.
Did you take the timing cover back off? There's those two passages on either side of the block right behind the water pump - what's in between - the water pump, right? (a common source feeding both banks) Is it a high flow type or have extra blades on the impeller? Are your pullies underdriven? Maybe a gasket is covering up the passages? Clutching at straws here.