And? Are not things in law substantiated on two [or more] witnesses?
I worry far less about abuse by governors [ATM] than I do about the feral FedGov; the way to beat governors is to act like, and demand that, the State's Constitution means what it says.
I've looked at all the States's Constitutions where I would be living, only this one have I given less than a good read-through... I only skimmed it. (Though that reminds me that I should give it a better read.)
I can't think of anything in law that is substantiated on two [or more] witnesses with no possibility for any further review.
Here's a hypothetical on top of your hypothetical - suppose that two governors declare it as an invasion, but then two other governors explicitly declare it NOT to be an invasion? Hell, suppose the other 48 governors all declared that it is NOT an invasion. Would the other 48 governors thus be traitors, simply because they disagreed with the two who declared an invasion?
Here's another question - suppose, after the two governors declare an invasion, that a majority of citizens in each of the states disagree and seek to recall the governors. If these citizens mentioned the invasion issue in their recall campaign ("Vote to Recall Governor X because he incorrectly declared an invasion"), would that be Treason?
My point is that your suggestion would give unchecked power to a very, very small number of people (potentially two). That is not what the Founders envisioned or intended the Constitution to allow.