Speaking as someone who is just getting my brother out of 2 very bad partnerships he got into with friends I will agree that you will not be friends in the end UNLESS you follow some steps.
1)Make sure they understand that this is a partnership not a friendship.
2)Regardless of how much you trust them get EVERYTHING in writing for proof.
3)If you are going to have control of the animals, i.e. you are going to be the one purchasing feed, paying the vet, etc. I would recomend first seeing if your supplier, vet, etc will bill your partner for their part of the bill, if not make sure you keep accurate records and send copies of all receipts when you bill them for their part. Our situation is different as we have animals that belong solely to me, partner animals and animals that belong solely to my partner on the same pasture. What I do is buy all the feed, hay, and minerals myself. I then figure out roughly what it costs to feed each animal daily. So for example if I figured it at $5 a day I would bill my partners $5 per day per animal they owned and $2.50 a day per animal they owned half of. I ussually stick with a good average number year round and I explain this to the partner and to people I may be keeping animals for...If I am charging $5.00 a day, it may only actually cost $4 a day some times in the year, but it may cost as much as $6 or more a day other times of the year, such as winter when I am feeding more, so it all averages out in the end. My vet bills me for my animals, a seperate account for the partnership animals, and my partner for their animals. I then bill the partner for half of the joint bill. For maintenance I bill the partner for half, unless there is proof that one of my partners animals broke something under extreme circumstances...at which time I bill them for all maintenance. I always include copies of receipts when billing for vet, maintenance and misc expenses.
4) You have to have a plan to disolve the partnership, will you sell all partnered animals and split the profit, will one of you buy the other out, or will it be a compination.
We did both ways...if we just put a price on animals that we were asking and I wanted the animal then I paid my partner half that price....one partner wanted money NOW so we took everything to the Auction Barn and split the cost...I simply went and bought back the ones I wanted.
It can be tricky ...but can work if done right...you just have to iron it all out BEFORE starting...its kinda like gentics...kinda hard to change once conceived.
Sorry so long but this is a fresh issue for me!