"A" and "B" refer to the landside stuff at MCO (check-in and baggage claim and parking) unfortunately so both airlines in "B" doesn't help one transfer between the two while staying airside. For your departing terminal in MCO, you have to look at which "Airside" you're in. As I noted above, UA is in Airside 3 and DL is in Airside 4, on opposite sides of the central terminal area.=, unless they've built some airside connector all the way across the terminal since the last time I was in MCO (6+ years ago).