It's also not really a scenario worth considering.
In order to cooperate, the miners/validators need to have full alignment on their goals. Which is very unlikely for Sybil attacks, which almost always destroy systemic economic value — including the value held by the attackers. You might be able to find e.g. a government who wants to do a Sybil attack to accomplish goal X, and are willing to burn down $N billion dollars of their own staked assets to do so; but can you find a bunch of other governments or large corporations who also want to accomplish that same goal X more than they want their own stake-value? And who can be trusted to coordinate and not leak the plan?
A Sybil attack is basically like a bank heist where the attacker(s) have to pull up to the bank in their own personal aircraft carriers (destroying said aircraft carriers in the process, from the hull damage of sliding along several city streets), without anyone realizing they're coming. You can maybe theoretically do it as one entity with full internal goal-alignment. It's much harder to do it as multiple entities.