In the case of a safeik peter chamor, the halacha is that the owner must set aside a seh which he is allowed to keep for himself -- hamotzi m'chaveiro alav ha'ra'aya, the burden of proof is on the kohen to establish that it really is a peter chamor and the sheep is his. What if the kohen has multiple safeik pitrei chamorim? Rashi holds that multiple sheep need to be set aside, one for each safeik. Tosfos however disagrees and writes that one seh suffices for all the sfeikos together.
The Shach adds an additional chiddush -- not only is that single seh good for multiple safeik pitrei chamorim, but it is even good to be re-used for a vaday peter chamor. In other words, if a sheep is set aside for the safeik peter chamor, the farmer can then give the same sheep to a kohein to redeem a vaday peter chamor.
The Minchas Chinuch (22:15) is not convinced, though he writes that to fully examine this issue would involve a tour through all of sha"s. What that means is that this is complicated, but worth the effort:
The conclusion of the famous sugya of tukfo kohen (B"M 7) is that a kohein has no right to seize a safeik bechor away from its owner and if he does, we take the animal back. The Rambam, however, paskens that if the kohein seizes the bechor, he has a right to keep it. Rishonim and Achronim galore try to justify the Rambam with the gemara.
The proof of the gemara is from a braysa that says if a farmer has 10 safeik animals, which the gemara reads to mean safeik bechoros, the farmer must take ma'aser from those animals because they belong to him and a kohen has no right to take them.
Rashba suggests that the Rambam (based on other sugyos) read that braysa as referring not to safeik bechoros, but rather to the sheep used to redeeem safeik pitrei chamorim. In the case of safeik bechor, the kohen does have a right to keep the animal he seizes; in the case of a sheep used to redeem a safeik peter chamor, he does not.
[What's the difference? The "classical" understanding of the Rashba is that the bechor is born into a state of safeik; the claims of the owner and claims of the kohen cancel each other out. However, the seh used to redeem the peter chamor definitely belonged to the farmer at one point in time. The kohen's safeik claim is not enough to disrupt the prior claim of chezkas marei kammah of the farmer. Figuring out whether or not this is actually what the Rashba meant is part of the fun of the tukfo kohen sugya.]
So a kohen can't seize this sheep used to redeeem a safeik peter chamor, but like every rule, there is an exception. The Rosh (and many other Rishonim) hold that tefisa, seizing something, means taking it without the owner's OK. If the owner willingly gave the sheep to the kohen for whatever reason, even if he then comes to the kohen and asks for it back, the kohen can keep it -- that's not called tefisa.
Getting back to the case of the Shach -- if a farmer has a safeik peter chamor and a vaday peter chamor, even if he gives his sheep to the kohen to redeem the vaday, what's to stop the kohen from saying, once he has that sheep in hand willingly given to him, that he is holding it as the pidyon for the safeik peter chamor and the farmer still owes him another sheep? And even if the kohen is not smart enough to make this argument himself, perhaps the fact that he could make such an argument is sufficient?