My first writing partner was Steve, with whom I produced the Munchkin’s Guide to Powergaming. He doesn’t work in gaming anymore, even a little bit, though he did assist with another early effort, Iron Empire (steampunk before steampunk really became a ‘thing’). He still games though, and we meet up with the rest of our group about every month and a half for a weekend-long session.
Otherwise I’ve worked alongside and with a lot of people in different games and books and to different levels of cooperation. Mostly though, I prefer to work independently, by myself, accountable only to myself. Once you’ve had that kind of freedom it is very hard to tolerate cooperation as it tends to read as interference.
For the longest time though, I have had a partnership with the artist Brad McDevitt, an industry veteran who sells stock art through me and has artwork in more of my books and materials than anyone else.