There's a line that Interstate compacts are constitutional if "consented" to by Congress. So it'd be like state govs form compact and send a letter to Congress, who acknowledges receipt. I'm sure it'd get challenged but I think the constitution is pretty clear that states select electors who can do whatever they want.

Of course if a state gov changes hands down the line, they can withdraw, but that doesn't screw anyone over, states will just go back to our current apportionment scheme