Marvel's Spider-Man 2 associate animation director James Ham revealed this in an interview with Danny Peña (via TCMFGames), explaining, "There are times where you can be swinging throughout the city and you can see the other Spider-Man, sometimes, actively doing something or stopping a crime."

Ham added that, not only will you occasionally see the other hero around town, but if you happen to catch them while they're in combat, you'll be able to team up with them. Same goes for if the non-player hero encounters you while you're squaring off against some baddies.

This makes me wonder if you can switch between Spiderman while actively fighting together

So…

that sounds real cool