Well, that's really a myth...or really more of a misunderstanding about the constitution. This is an area of the constitution that is a confusing mess frankly.
There is NO constitutional prohibition against the President and Vice President being from the same state.
Originally, electors in each state didn't for a President and Vice President. They cast votes for two people to be President. Whoever got the most votes became President and the runner up was Vice President. There is a prohibition against electors voting for two people, both from their own state. The founders were trying to preclude a large state like New York from having too much power and voting for two people from their own state. However, nothing in there precludes all of the other (now 49) states from voting for those two people who are both from New York.
The flaws in the original system came to light pretty quickly and the 12th amendment was passed. After that, electors make two separate votes...one to pick the President and one to pick the Vice President. However, given governmental idiocy, they left in the clause prohibiting electors voting for two people from the same state.
All that said, if Trump and DeSantis ran, there would be no issue with electors from 49 states voting for them. In Florida, there's a gray area that the courts might wind up having to weigh in on. Could the electors in Florida vote for a President Trump and Vice President DeSantis...probably not, but not certain how it would play out in court....if it went there. They could certainly cast votes for Trump as President and then vote for anyone else as Vice President. That would kick the can into Congress' hands. When they met to count the electors votes, Trump would have X votes for President, but DeSantis would have X-30 votes for Vice President since he wouldn't' have Florida's votes. If the election were a blowout, it wouldn't matter.
If the election turned out to be really close, you could in theory wind up with President Trump and Vice President Harris (or whoever was the VP candidate on the Democrat ticket). Wouldn't that be a hoot!!!