There is a big difference between pidyon haben and yerusha. Pidyon haben is peter rechem – 1st born of the mother. Pi shnayim for yerusha is reishis ono – 1st born of the father. A man with multiple wives could end up being podeh a son multiple times, but only one will get pi shnayim in the yerusha. Therefore, I don’t think we can compare the halachos of pidyon haben with yerusha. As for taanis bechorim, I’m not familiar with the halachos, but lav davka that the rules are comparable with pidyon haben and/or yerusha.
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