What is the two wrongs make a right fallacy?

Justifying a wrong action by pointing out that the other party did (or would do) the same thing.

The pattern: B did X to A (or would), therefore it's acceptable for A to do X to B — even when A's action isn't actually needed to prevent B's. It fails because a wrong act doesn't become right just because someone else also commits it.

"I kept the extra change the cashier gave me by mistake — they'd have charged me if I'd underpaid, so we're even."

The store's hypothetical behaviour doesn't make keeping money that isn't yours acceptable. (Genuine self-defence or retribution is different: striking back to stop an ongoing attack can be justified — the fallacy is retaliation when no such necessity exists.)

Tip: "They started it" / "they'd do it too" excuses revenge, not the rightness of the act itself.