I agree with you in that these arrangements have to work for both parties, or they don't work at all. I also know what you are saying about stockpiling embryos and only having so many recips that you can put embryos in. I am in the same boat. I have tried a couple deals with some neighbours, but they have resulted in mixed results. If you happen to find a good cooperator herd, make sure you hammer out a solid deal that is fair to both parties. Everyone will have slightly difference circumstances so it is hard to come up with one set of rules for the splitting of the calves and the costs.
Personally for me, the near perfect scenerio would be a young breeder who is trying to get established, who would like to share in the calves. I would pay all implant costs and provide the embryos. He would provide the recips, the management and the feed, including creep feed for the calves. We would decide on a date that they would be divided between us on a 50: 50 split. I would also agree that if there is up to 2 male and 2 female ET calves that are exceptional, to take them out of the split and continue to jointly own them until they are sold or their show career is over. For the split, either myself or the cooperator herd owner, would split the calves into two groups, then the person who did not split them, would get first pick of a group of calves. It could also work to alternatively pick calves until they are all diviided, and the following year, reverse who gets first pick.
For me, I think it would work well as it limits the cash payout required for either party. It helps me not have to purchase more recips, and it helps the recip owner to establish a breeding unit. Just make sure you don't get greedy and also don't go into a deal if it looks like the cooperator is going to get greedy as well. Just move on and try to find another. It just ain't worth the hassle.