Meilah 7b - R' Yochanan's proof and b'vas echas

On 7a, Rashi explained the case of the two korbonos with acharayos b'bas echas by having two kohanim do the shechita so that "neither one preceded the other". Tosefos offered a version of that type by saying the shechita was with "a long knife". In that case, זה אחר זה would be any time the shechitos were not at the same moment, even if the result is that we have blood from each ready to do zerikah, "רצה מזה זורק רצה מזה זורק".
On the other hand, Tosefos themselves explained בת אחת as any case where the second shechitah took place while the first one's zerikah hasn't been done. זה אחר זה would mean the first one's zerikah was done before the second one was shechted.

Now here on amud beis, the gemara talks about offering the אימורים of one korbon without zerikah, then doing zerikah from the second korbon. That is clearly going to be the בת אחת case according to Tosefos - they ask about this, it is obviously going to need an explanation: How can R' Yochanon bring a proof about how זה אחר זה would work, from the case that is בת אחת? (See Tosefos for how they explain it.)
According to Rashi, on the other hand, this case could be either בת אחת or זא"ז, depending on whether a long knife etc. was used. It would seem that he could understand that the case is זא"ז and then R' Yochanan's proof will be clear and simple.
Thus, I don't understand why Rashi asks, ד"ה אמר ר' יוחנן כו' ואי קשיא לך כו' מ"ש, where he asks why בת אחת works here where it didn't earlier - essentially Tosefos' question. Rashi doesn't seem to need an answer, only Tosefos.

I'm wondering if this is similar to the issue with Rashi on 4a, ד"ה אי הכי, where he explains the gemara's question in a way that makes sense for Tosefos, not for him - for him there should be no such question. The mefarshim discuss it there.
So I'm wondering if the answer is really that "Rashi" on Meilah is not a single consistent peirush, but a patchwork of different mefarshim. It's already known that most of it is just quoted from Rabbeinu Gershom, probably some talmid of his. And the mefarshim already move his words around a lot. Could be the really hard parts go according to a different opinion that other parts. Has anyone seen such a suggestion?