It is well accepted that the shiur to combine eating into one act is כדי אכילת פרס. However, there is a contradiction between two statements of Rashi (d.h. cheilev, and She'im) how this is measured. The first Rashi implies that it is calculated by their being a כדי אכילת פרס in between the two eatings. Meaning, that the time that one is actually eating doesn't count, therefore as long as the time in between is less than a pras the two acts of eating combine together. However, the second Rashi implies that one must consume a kezayis within the amount of time it takes to eat a pras, implying that it is measured from the start of the first eating until the conclusion of the eating. Which one is it?
The Rashash cites a Sefer Be'er Avrohom (author of maskil l'eisan) - HERE, who asks this question on Rashi. He points out that both the Mishna in Krisus (3rd perek) and Rashi in Pesachim 44 imply that for something to be considered one act of eating, the entire kezayis must be consumed within an amount of time of כדי אכילת פרס. Whereas Rashi in Menachos 75b implies that so long as the amount of time in between is less than כדי אכילת פרס, the the two act of eating combine to a kezayis. The sefer Be'er Avrohom makes a distinction between an aveira and a mitzvah. He suggests that to be in violation of eating something forbidden, one must consume the entire kezayis in a כדי אכילת פרס. But, when one is eating a kezayis of matzah, so long as there is no pause in between his achila that is longer than כדי אכילת פרס, it all combines to enable him to be yotzei his mitzvah, even though it took longer than כדי אכילת פרס to eat the kezayis. With this he explains the Braisa in Pesachim 114b that if one eats a half of kezayis of maror and then another half, they combine so long as there isn't a delay in between that is longer than כדי אכילת פרס. This doesn't easily answer the contradiction in Rashi in our sugya, but the Be'er Avrohom suggests that the first Rashi is trying to say a chiddush that even if the delay in between the two eatings of cheilev is longer than כדי אכילת פרס, they combine together to be considered one eating.
The Rashash rejects the distinction of the Be'er Avrohom to say that mitzvos are different than aveiros. His proof is from a Yerushalmi in Shavuos (perek 3) cited by Rif and Rosh (end of Pesachim) that if one makes a shavua not to eat matzah on the night of pesach, it is a shavuas shav, so he gets malkus and eats matzah. The Yerushalmi assumes it is impossible to fulfill the mitzvah of eating matzah without violating the aveira. Why is this impossible? One can eat a kezayis with a break of just under a כדי אכילת פרס in between each half kezayis, thereby fulfilling the mitzvah of matzah but not violating the shavua since there will be more than a כדי אכילת פרס from the beginning to the end of his kezayis.
It is also difficult to understand what rationale there can possibly be to differentiate between a mitzvah and an aveirah. For some reason the assumption is that for a mitzvah, so long as one is eating the entire time, we don't care how long it takes him to eat the kezayis, whereas for an aveira it only qualifies as a violation of eating a kezayis when one eats a kezayis within כדי אכילת פרס.