In the past I have played in a number of bands at the same time. Two were as a permanent member, and the rest were guesting to make up numbers. I found that it worked because I was honest about who I was playing for and when. All the bands understood that I took bookings on a "first come, first served" basis and I stuck to my word. None of the permanent bands had shed-loads of work so there were not too many clashes, and if there were then it was up to the rest of band members to decide whether to a) turn the booking down, b) do it with a smaller line-up, or c) recruit other musicians to sit in. I understand that the busier and/or more professional the band is, the more problematical this would be - the Stones would have felt a bit miffed if Mick Jagger had kept bunking off to sing with the Beatles. However, the arrangement worked for me and kept my bookings up to a reasonable level.