Possible outcomes of Tarquin and Xykon recognizing eachother:

1) Old friends.
"Hey, Xykon, old friend..."
"I don't remember you. Meteor Swarm."

2) Old enemies.
"Oh no, it's my old adversary!"
"I don't remember you, but if you insist, Meteor Swarm"

3) They don't recognize each other
"Hey, who's that guy?"
"Allow me to introduce myself. Meteor Swarm."