The Mishnah described the order of tithing as first Maaser Rishon, then Challah and then Maaser Sheni...

(R. Yona citing Zeira): There is no prohibition to delay the giving of Challah, so one can tithe first and then separate Challah.

(R. Yosi citing R. Zeira): The Mishnah was not said in order. (Since Challah is like Terumah, it must be separated first.)

Question: Aren't we discussing Demai - didn't R. Ba son of R. Chiya say in the name of R. Yochanan that the Mishnah specifically refers to Demai (when allowing tithing before separating Challah), but if it's definite Tevel, the Challah must come first...?

R. Chiya's Baraisa taught that the Mishnah also refers to definite Tevel and based on that, the two Amoraim disagreed...

(R. Yona citing Zeira): There is no prohibition to delay the giving of Challah, so one can tithe first and then separate Challah.

(R. Yosi citing R. Zeira): The Mishnah was not said in order. (Since Challah is like Terumah, it must be separated first.) Rather, you could take Challah before Maaser Rishon, but Rishon comes first because it's obligation came first at the threshing floor (but the Challah obligation only came when it was made into to dough).

Question: But the obligation of Maaser Sheni also came at the threshing floor (before that of Challah)?

Answer (R. Matanya): Really Challah should come first, so why is Rishon first? Because part of the Maaser will become Terumas Maaser, about which the pasuk says 'first' - so although Challah is also called 'first', the Maaser Rishon precedes it because its obligation came first. However, Maaser Sheni comes after Challah because although its obligation came first, it is not called 'first'.

Question: If so, Challah should be obligated in Maaseros, but Maaseros should not be obligated in Challah, as whichever comes first, the other is obligated in it, but not vice-versa...?

Answer: But do we say this? Doesn't the Baraisa teach - Tevel dough - whether Challah preceded Terumah or Terumah preceded Challah, whatever was done is valid. The Kohen may not eat the Challah until he separated the Terumah, and vice-versa. (The Gemara doesn't resolve the discrepancy.)