Just my experience, but partnership heifers usually end up doing more harm than good. Face it, unless she turns out to be a great and at least fairly prolific donor, she's prob'ly not really making money for anyone. I know that's always the goal when you buy or sell part of a heifer, but when you consider how few of them ever make it there, and how many ways it can go wrong, I'd rather just sell the heifer and retain rights to a couple of flushes later on - or keep her and breed her myself and sell that interest later when (if) she makes it to proven donor status.
If you do decide to sell half (or whatever percentage) of a heifer, though it certainly couldn't hurt to get it all down on paper, I'd still be careful to only do it with someone you trust and believe you can get along easily with.