Hi Bob.
You do all the heads at once. The key to demagnetizing is to plug in the demagnetizer at least 3 feet away from your tape deck, then bring it SLOWLY toward the deck and head stack. By SLOWLY, I mean SLOWLY - take 5 seconds to bring the demag from where you plugged it in to the head stack. Pretend you are defusing a bomb and any quick move will blow you to bits. Once you get it to the head stack, move it around each head and the tape guides. Again, make all moves deliberate and slow. Then move the demag away at least 3' - again, take at least 5 seconds to do this. Unplug the demag only when you are at least 3' away.
John C.