It doesn’t seem to happen like it used to but back in the day all the hep kids were dropping science in each others’ backyard:
Sluggy Freelance and User Friendly: AJ drops in on Riff; and Torg gets a job with Columbia Internet.
User Friendly and Superosity: Dust Puppy visits Superosity-land.
Bobbins and Waiting For Bob: The Bobbins crew met up with Waiting for Bob’s Bernie and Sean.
Bobbins and Goats: Jon from Goats meets Holly from Bobbins. Small sparks gain a little altitude. Later Jon and Philip actually make it to England.
When I Grow Up and Waiting For Bob: Zoe investigates Sean.
Bobbins and Superosity: Holly and Shelley visit Boardy and Bobby goes to England. Somewhere along the way Professor Ash shows up.
When I Grow Up and Diesel Sweeties: Roger finds true robot love! (not entirely sure where in the DS archives its half is)
I’m sure there’s lots more I’m forgetting – feel free to post more classic crossovers below. Thanks!