On the last line of 32b, the Gemarah asks why did R. Yohanan not concur with Ulla.
I don't understand the question or answer. Since Ulla's explanation was based on a gezeirah shavah of "tachat, tachat" and since R. Yohanan learned something else from that gezeirah shavah, why does the gemarah need to give an additional reason as to why he rejected Ulla's view?
When you write, "since R. Yohanan learned something else from that gezeirah shavah," I assume that you are referring to what Tosfos says in DH v'Rebbi Yochanan. Tosfos writes that Rebbi Yochanan admits that the Gezeirah Shaveh of Tachas-Tachas is a valid Gezeirah Shaveh. However, he holds that it is only applied in other Pesukim, and not in this Pasuk. You ask, that if so the Gezeirah Shaveh is "used up already," for Rebbi Yochanan, so of course it cannot apply here.
I don't think that is true. Once we know that the word "Tachas" can be used as a Gezeirah Shaveh, it can be applied any number of times, to any number of verses. The Gemara is asking why Rebbi Yochanan did not apply it here, although he applied it elsewhere. The original answer of the Gemara is that he did not feel it logical to apply it here. The final answer is that there is no extra word Tachas here (i.e. it is not Mufnah), since he uses Tachas to teach something else.