An enemy spawns at Zoey's location and she uses her cross to deal one damage to it as it engages her, exhausting the cross in the process. During her turn Zoey successfully evades the same enemy, causing it to disengage and exhaust at her location. When cards ready, do they all ready simultaneously, or do the players choose the order? Put another way, can Zoey ready her cross before the enemy readies and engages her again? I can't find anything specific in the rules or FAQ, but I'm guessing it's probably simultaneous. In which case the cross cannot be used when the enemy engages again.

I can’t imagine it would let you, I think the rule of thumb is the worst possible way to do something is correct lol (I forget what it’s called, the dark rule? The black rule? Idk) but I am unsure

The Grim Rule. However I am asking if there is a specific ruling in place that covers this situation.

It is "the Grim Rule", but that rule is not (necessarily) a guiding philosophy of the rules. The Grim Rule just says that if you are unsure about a ruling in the middle of the game and cannot find the answer in a reasonable amount of time, don't waste time arguing/worrying about it at that moment, just pick the worst outcome and keep playing. I suspect the pertinent rule here is on page 25 of the Rules Reference:

So all cards get readied at the same time.

Thanks Eeyogre. That's what I was looking for.

1 hour ago, psmythirl said:

[...]I'm guessing it's probably simultaneous. In which case the cross cannot be used when the enemy engages again.

@Eeyogre is absolutely correct that they will ready at the same time.

However, a key point that has not been addressed is that this means you can use Zoey's Cross on the enemy when it engages during the Upkeep Phase.

Zoey's Cross states: "[Response] After an enemy becomes engaged with you, exhaust Zoey's Cross and spend 1 resource: Deal 1 damage to that enemy."

By the time you would trigger this (the point immediately after the enemy engages you), Zoey's Cross has already been readied and you can trigger its ability.

Would you need a Player Window to trigger the cross? I don't see one after "Ready each exhausted card." in the Upkeep phase until after "Each investigator draws 1 encounter card." in the Mythos phase. But I have been known to be wrong about this type of detail.

32 minutes ago, Eeyogre said:

Would you need a Player Window to trigger the cross? I don't see one after "Ready each exhausted card." in the Upkeep phase until after "Each investigator draws 1 encounter card." in the Mythos phase. But I have been known to be wrong about this type of detail.

No. It's a response ability, not a free triggered ability, so player windows are irrelevant - all you need is to meet the triggering condition (here, becoming engaged with an enemy) and to be able to pay the costs (here, exhausting Zoey's Cross and paying one resource).

9 hours ago, Allonym said:

By the time you would trigger this (the point immediately after the enemy engages you), Zoey's Cross has already been readied and you can trigger its ability.

The only relevant rule I can find is (Rules Ref. pg. 10 - Enemy Engagement) :

Quote

... if an exhausted enemy at the same location as an investigator becomes ready, it engages as soon as it is readied.

My reading of this, is that engagement takes places after readying, and since readying is simultaneous, the cross can react to the engagement.

5 hours ago, psmythirl said:

The only relevant rule I can find is (Rules Ref. pg. 10 - Enemy Engagement) :

My reading of this, is that engagement takes places after readying, and since readying is simultaneous, the cross can react to the engagement.

Precisely.