I typically pay birth back ups hourly, usually $50 per hour with a minimum of $200 at night and $100 during the day (that's in the case of going to a birth for an hour, I want to compensate them for the time it takes to put their life on hold for such a small amount of time!). I also have caps, usually around $1000 - $1200, unless the birth is really long, then I play it by ear. I also want to make sure I am getting compensated for all of my support around the birth, like being on call and doing prenatal visits.
In the event we both attend the birth (if the birth was five days I am definitely calling back up at some point) I divide up the hours we both spent at the birth and pay us the same hourly rate.
So If I was paid $1800 for a birth, and the back up went to the birth for 20 hours, they would get $1000 and I would get $800.
My goal with back ups is to make them feel like they really want to provide support for me - if you aren't compensating your back up well, it could be really difficult to find someone at 3am.
Some doulas find it easier to provide each other back up and not exchange any money, figuring it'll all even out in the end. This is also a great idea, however, you always want to make sure your back up list is long - I've called several people before and not one could make it. Make sure you have lots of back up options.
There are so many variations and what ifs to back up and no "right" way to do it - just make sure you AND your back up both feel good about the payment in the end.