Agnes, it has already happened a couple of times. The first was a two-part comic. Each of us did one part on our own page (there’s a link to part 2 at the end of part 1):

Cave Of Treasures

And recently Roy provided the punchline for one of my comics:

Strange Pirates

I would love to do another. But Roy is such a big star now, and I hate having to go through his lawyers and management to ask him anything.