The gemara has a rule in many places that although a positive mitzvah (aseh) can push off a negative mitzvah (lo ta'aseh), anytime a mitzvah has a lo ta'aseh and an aseh we don't allow another aseh to push off both the negative and positive mitzvah. The example is the mitzvah of covering the blood of a bird when it's shechted where there is an aseh to cover the blood, but if done on yom tov it would violate an aseh and lo ta'aseh, so we don't allow one to do the mitzvah of covering the blood on yom tov.
However, the question is raised whenever we have a situation of aseh trying to push off aseh and lo ta'aseh, whether we allow the aseh to push off the lo ta'aseh just not the supporting aseh, or do we say that since the aseh can't push off the opposing aseh, it also can't push off the opposing lo ta'aseh. The Riva quoted by Tosafos in Chulin 141a holds that in a situation where there is an aseh opposing a lo ta'aseh and and aseh, if one would fulfill the aseh thereby violating the lo ta'aseh and aseh, they would not get lashes for the violation of the lo ta'aseh because the aseh effectively pushed off the lo ta'aseh, just isn't powerful enough to push off the opposing aseh. In my sefer, Nasiach B'chukecha (page 214) I discussed this issue and showed how it is a machlokes rishonim. In the additions to my sefer, I pointed out that Tosafos on today's daf seems to disagree with the Riva and holds that when we have an aseh up against a lo ta'aseh and an aseh, it would not even push off the lo ta'aseh so that if it is violated there would even be malkus for the violation.
Tosafos D.H. Minayin, asks why do we need a source to teach that the prohibition of shechting a mother and child on the same day would apply to kodshim, the fact that it would be written without any qualification would automatically make it apply to kodshim? Tosafos suggests that perhaps we need the pasuk to say that even if there is a mitzvah that must be done with this animal such as korban pesach, you cannot shecht it if the mother has been shechted earlier that day. Meaning, without a special source we would allow the mitzvah of Korban Pesach to push off the prohibition, but now that we have a source that it applies to kodshim we wouldn't allow the mechanics of aseh pushing off a lo ta'aseh to take place. Tosafos rejects this answer because shechting a child the same day as the mother would be a violation of both an aseh and a lo ta'aseh so even without a special source we would never have allowed the aseh of korban pesach to push off both an aseh and a lo ta'aseh that forbids shechting the mother and child on the same day. Tosafos seems to understand that whether we had a special pasuk declaring that one cannot shecht the child on the same day as the mother to fulfill the mitzvah of korban pesach, or didn't have a special pasuk, the result would be the same since the rule of אין עשה דוחה לא תעשה ועשה wouldn't allow the violation. Now, if the Riva is correct that when we have a situation of אין עשה דוחה ל"ת ועשה we wouldn't give lashes for the violation because the aseh would indeed push off the lo ta'aseh, just not the supporting aseh, then we would still require a pasuk to say that the issur of אותו ואת בנו doesn't apply to kodshim. Without a special pasuk, if one were to shecht the child the same day as the mother, he wouldn't get malkus. Now that we have a special pasuk teaching that even by kodshim animals that are needed for a korban the prohibition applies, there would be malkus for the violation. The fact that Tosafos maintains that the rule of אין עשה דוחה ל"ת ועשה is sufficient even without a special pasuk, implies that the rule of אין עשה דוחה ל"ת ועשה alone would also allow us to give lashes to one who violates, which is against the Rivah.