I had an immediate flashback to the Cat Wars of a few years back between Becket and Nishka - Becket looked *just* like he did before, and I can't go through that again. Especially since I'm sure that Henry would hold up his end of the fight better than the old lady did. So I caught Henry, handed him to Bob, and caught Becket myself. Every hair on both cats was standing on end (which was a fairly freaky look.) Then I took them both into the living room and gave them treats. Lots of treats. Look - good things happen with the other cat!
It appears to have worked. Becket backed off, and Henry wasn't really that irked to begin with. The old man is just *cranky* about other cats, and apparently *really* susceptible to non-recognition aggression in his older age. I'm glad that I appeared to have headed it off at the pass, and my dinner was even still warm when I got back to the table.
They still both kind of explode at loud noises, and neither of them goes very far from the front door, or stays away from it very long. Watch out strange cat - they're waiting for you.