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.
That's not too hard to imagine/explain as reality: lay out a map of the US at your next picnic and smear honey on TX and NM — letting all the ants represent illegal aliens, tell me which states are invaded? (That is, the lack of invasion on other states does not invalidate the state of invasion on those other states.)
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?
IMO that would depend on if the State's Constitution either (a) explicitly gave the Governor that authority, or (b) explicitly gave the Citizens the right to recall him. — I'm assuming, also, that they'd also have availed themselves of any possibility of demanding redress.
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.
It's not unchecked power, obviously. (It is rightly subject to the State's own Constitution; that is what the 9th and 10th Amendment are about.)
Do you think this feral/federal government is!?