Speaking of R’ Yaakov Emden’s position, let’s say in my above story the only issues at hand were the two pidyon habens, and the guy has 10 selaim. The Gemara in Kiddushin (29a-b) says:
??? ???? ??? ????? ???? ????? ??? ???? ???? ??? ????? ???? ??? ????? ??? ????? ?? ???? ??? ???? ??? ???? ??? ??? ????? ??? ????? ?? ???? ????? ??? ??? ????? ??? ???? ???? ??? ???? ???? ?????? ????? ?? ????? ???? ????? ??? ???????? ???? ??? ????? ?’ ????? ??? ???? ????? ????? ?????? ???? ???? ???? ??? ???? ????? ????? ??? ????? ??? ???? ???????? ?????? ????? ???? ???? ????? ???????? ??? ?????? ???? ???? ?????? ???? ?????? ????
So it’s clear that his own pidyon haben as priority over his son’s. Now let’s say there are two kohanim. The guy gives five selaim to one kohen for his own pidyon haben. Now he has five selaim left. Should he use it for his son’s pidyon haben, or should he use it to do his own pidyon haben from the second kohen. According to R’ Yaakov Emden he still has a chiyuv for his own pidyon haben, and the Gemara says that his comes before his son. So lichora, the guy should redeem himself twice and not redeem his son. However, there is a good argument against this (which is the same argument that I applied to an esrog at http://www.theyeshivaworld.com/coffeeroom/topic/techeiles/page/5#post-398593 ) namely that legabay the son’s pidyon haben it’s a safek d’oraisa (if the kohen is a valid kohen) whereas legabay the father it’s only a sfeik sfeika (1- he might have already been redeemed from the first kohen and 2- the second kohen might be a valid kohen) so the chiyuv of the son should supersede the chiyuv of the father.
Either way, in a stam case of a pidyon haben, every additional kohen you go to would be a sfeik sfeika and then a sfeik sfeik sfeika and so on. So the position of R’ Yaakov Emden is a clear support to the Mutzal M’eish which I quoted at http://www.theyeshivaworld.com/coffeeroom/topic/techeiles/page/5#post-398814 that by an asei, even a sfeik sfeika is lechumra.