The Shield of Abraham (Siman 494) writes regarding the eating of matzah: "An olive's bulk from each. Also, it implies in the Tur that one needs an olive's bulk for a piece that one is making the blessing over, and similarly in the ways of Moses (Sign Aleph): in the Trumat Hadeshen (Siman 138) it is written that one must eat the olive's bulk together and swallow it, and so wrote the Beit Yosef (Siman 486, D"H Veyikach) in the name of the Tosafot (Chullin 23b D"H Chelko), see there. It can be understood from his language that when one swallows one olive in one go, as also implied in the Trumat Hadeshen there, which states that we say (Pesachim 115a) 'wrap three together in one go': the Mordechai raises a difficulty (there 38, 3), how does the esophagus hold all that, and an olive is half an egg. And it can be said, that when it reaches the esophagus it is mushy, and therefore it holds all that, this is the reasoning. From here it is clear that one should not swallow the matzah olive bit by bit, for if not, it would not be difficult at all, as one could easily fit all three olives in their mouths, and drink in one go, and swallow bit by bit, rather it is a mitzvah to swallow them together to praise, and the same is true for us with one olive of matzah or maror, see there, it seems that he is of the opinion that it is enough when one swallows one olive in one go. And what is written in the Shulchan Aruch to eat them together, meaning to put them in the mouth in one go. And the Maharil (Laws of the Passover Haggadah page 115, variant texts letter Y) writes, that one can eat bit by bit, as long as one does not pause in their eating more than the size of a piece, but in section 6 it implies that specifically after the fact one fulfills the obligation, but ideally one should eat together, and so it implies in Chullin end of chapter 7 (23b), that one must swallow together. However, in the case of maror, one can rely on the Maharil (there page 121) if it is difficult for him to swallow together."

And the Mishnah Berurah (there, paragraph 9) concludes: "And later authorities agree that one only needs to put both olives in the mouth at once and chew them, but one is not obligated to swallow them in one go, rather it suffices to swallow an olive's bulk approximately in one go, and then swallow the rest. And after the fact, even if one swallowed the olive bit by bit, one fulfills the obligation, as long as one did not pause from the beginning of eating until the end more than the time it takes to eat a piece."