88a Counted On Two Pesachs?

The Mishna says that if two caretakers of an orphan both shecht a Pesach for him, the orphan may decide from which one he will eat.

 
To execute the legal action of becoming part of a Pesach Chabura, one needs to consent. Now, if only one caretaker made him part of his Pesach, perhaps we could assume consent, but since both of them did so, we need explicit consent. Therefore, the Gemora assumes that after the orphan decides to eat from one of them, we will extrapolate consent retroactively.

The Gemora asks on this, can we then derive from here that we may execute a legal act by way of retroactively determined consent?

The Gemora answers that in the Torah, it says “שה לבית”. Rashi explains that this means to tell us that a father may add his children to a Pesach without their consent.

It is seemingly not clear how this answers the question; even if the orphan doesn’t need to consent, how will we determine which Pesach he is on?

The Baraisa on 88b states that one may not be registered on two Pesach’s. Rashi explains that this is because we have no way to determine which Pesach he desired to eat from at the time of Shechita. We have no way of determining which one he consented to use as his Pesach.

We can therefore explain our Rashi, that since a minor doesn’t need consent, he can therefore be numbered on two Korban Pesach’s. We have no need to determine his desire retroactively, because his desire is irrelevant.