me and a mate bought an old 328i and trailer for track use - the original plan was that everything was a 50/50 split - we did 6 days together, and shared the driving on the day as equally as possible.
Luckily we were in the situation that the car was so cheap that if we did bin it, we'd just split the losses and buy another one - we didnt have anything in writing though.
In the end we had to sell it as my other car at the time was costing me more and more money so i could afford to keep it going, also (although i didnt say this) my friend was charging out petrol at a business rate per mile - admittedly he was doing all the towing, but if i was doing any towing, i would have just charged for petrol, rather than petrol plus wear and tear on his car.
Another point was he was keen to buy things that i was less keen on - such as new tyres for the trailer, new ratchet straps and a couple of other bits and bobs.
I'd do it again, but as i say, make sure you both know what you want before you start, and also i'd plan out what days you'd wanna go and how many days you'd be doing a year, so you have an idea what the costs are going to be.