I've tried it once or twice, it wasn't for me.
Or maybe it was just the wrong design partners, or the wrong time in one of our lives for a partnership.
Every time I've tried to partner with someone on a project, they've flaked on me, I've flaked on them, or we've just had creative differences on the project and walked our separate ways.
It was probably about 20 years ago when I had my most successful game design partnership with Dave Chandraratnam. We wrote freewheeling chaotic games for the local convention scene. Pushing boundaries in both narrative, and the way games were played. We lost contact when work and varied social commitments pulled us apart, but occasionally I wonder what happened to Dave.
After that, I've had a few attempts to collaborate on comics or games with a dedicated partner, but my only successful attempt at partnership has been the 15 years with my wife... and every time I've tried to game design with her has been tense to say the least.