It would be more than two planes crossing the same space. There would be many and they would fill the sky. Dozens of them. You would notice because it would be unusual to say the least. I live within sight of the various trails caused by activity at Vandenberg and have seen some very strange but explainable things from there. I had never seen this before, or since for that matter.
It's just the Air Force playing Tic Tac Toe. They use X's more than anything else because it's hard to get an F-16 to flight a tight enough loop for O's. It's not as bad as when the Marines play Hangman with crop circles by using Harrier exhaust. :D