Thanks folks for your input. I asked a couple of local producers here what they do.

One decides on a case-by-case basis: "We don't have a 'policy' about that. Sometimes we have guarantees (which are always vs. 85% of the door, whichever is more), sometimes we have an opener with a specified amount and the main artist gets the rest. I think you need to do it on a case by case basis. I can see both points of view, but people playing in a band know that they won't make as much as playing solo, but they enjoy it more! Another variable...maybe one act came from out of town and has travel expenses."

The other tends to split 50/50: "We split by act, though occasionally I'll do another arrangement if we have a solo with a quartet or something like that. Particulars, I think, aren't so important as making sure all is explicit and up front--contracts aren't a bad idea, either... Hope this helps-"

So I guess the bottom line is to be up front and explicit in the contract. My inclination is to split 50/50 unless there is a good reason to do otherwise (but then not necessarily on a strictly proportional basis).

Cheers, S. in Seattle