Oh, what a wonderful observation! I'd never noticed that, and it is just so much fun! Where I live, we usually say "X"s and "O"s in tic-tac-toe, not "crosses" and "circles," so perhaps that's why I've missed the association. But Nick wouldn't miss it, would he? Especially if Schanke were drawing his "X"s as "+"s.
What is the usual Canadian expression for the markings in tic-tac-toe?
Almost all tic-tac-toe games between adults tie, in my experience. It's very easy to maneuver your opponent into a tie, and very hard to beat someone who knows how to maneuver a tie, if they're paying the least attention. I think I'm going to interpret Nick and Schanke being evenly matched in tic-tac-toe as a symbolic affirmation of their partnership being one of equals well-matched, if very different.