I get SO many offers from other musicians to "work together". Sax men, guitar players, drummers, singer, etc. While I am a little flattered, the real reason they are wanting to do this is because I'm working, and they, for the most part, aren't. In other words, they need me to make a duo, but I don't really need them. Even if I were able to double my price, it is still a lot harder to accommodate another picker than it is to do a solo. Also, most would want an even split, even though I would be making the bulk of the music, supplying the pa, getting the bookings, etc.
I do enjoy interacting with other musicians, I just have found I make more money going it alone, and with less work involved. When a job demands an extra body or two, I just hire a sax player of guitar player for a set fee, usually $100. and keep the rest myself. It's my profession, but a sideline for most musicians. I've found I can get really talented sidemen for very little money. (But I do like to be fair with them).
So when I am approached by another musician, I now tell them, "Sure, my minimum price is $___. Let me know when you book something for us and I'll check my calendar."
DonM