This is because they are both from Florida, and the Constitution does not allow electors in the electoral college to cast both of their votes (for president and vice president) for a candidate from their home state.
If Trump and DeSantis run together, one of the two will not get Florida's electoral votes.
It's literally the first part of the first sentence of the 12th Amendment to the United States Constitution
The Electors shall meet in their respective states, and vote by ballot for President and Vice-President, one of whom, at least, shall not be an inhabitant of the same state with themselves;
Only one vote cast by an elector may be for an inhabitant of the same state as the elector. If the Floridian elector votes for Trump for President, the elector may not vote for DeSantis for Vice President (or vice versa).
DeSantis WILL NOT be Trump's running mate
This is because they are both from Florida, and the Constitution does not allow electors in the electoral college to cast both of their votes (for president and vice president) for a candidate from their home state.
If Trump and DeSantis run together, one of the two will not get Florida's electoral votes.
That’s not how it works
It's literally the first part of the first sentence of the 12th Amendment to the United States Constitution
Only one vote cast by an elector may be for an inhabitant of the same state as the elector. If the Floridian elector votes for Trump for President, the elector may not vote for DeSantis for Vice President (or vice versa).