The gemara (according to Rashi) has two versions of R. Yochanan as to whether the concept of כדי אכילת פרס applies to אבר מן החי, like it would by all other issurim. On the top of the page the gemara assumed that it would not apply, and on the bottom of the page when it deals with eating a half kezayis and then another half kezayis, assumes that it does.The rationale that אבר מן החי would be different and not have this halacha l'moshe misinai is that even inedible foods combine to the shiur of kezayis. Since אבר מן החי is a chiddush, the halacha l'moshe misinai that allows us to combine all eating within כדי אכילת פרס wouldn't apply to a chiddush case. Without the concept of כדי אכילת פרס the gemara assumes that the entire kezayis must be consumed בבת אחת, but then enters a discussion as to what would qualify as בבת אחת - at one time.
There seem to be three opinions in the gemara what would qualify as בבת אחת, and is dependent on the definition of אכילה. R. Yochanan considers eating to be a function of הנאת גרונו - the mouth. Reish Lakish considers eating to be a function of הנאת מעיו - the stomach. Therefore, R. Yochanan will say that as long as there is a kezayis in the mouth at one time, it is considered בבת אחת, whereas Reish Lakish will say that a kezayis must be swallowed at one time (Rashi explains that it is not possible to chew a kezayis and swallow it all at once since it will naturally begin to slip down his throat, so the only way to swallow a full kezayis would be without chewing. Rashi also seems to hold that swallowing something which is normal to chew wouldn't constitute דרך אכילה, therefore the gemara is forced to make the case of גרומיתא זעירתא which is normal to be swallowed without chewing).
Within the opinion of R. Yochanan that it only qualifies as בבת אחת if a kezayis enters the mouth at the same time, Rashi and Tosafos argue whether we require the kezayis to literally be one unit when it is put into the mouth (tosafos), or whether it can be cut in half so long as there will be a full kezayis in the mouth at one time (rashi). Similarly, they will argue in the opinion of Reish Lakish. Rashi should only require a kezayis in the stomach at the same time, even if it were swallowed in parts (this is actually tosafos question on rashi because reish lakish seems to hold that a full unit of kezayis must be swallowed at one time), whereas Tosafos requires a full unit of a kezayis to be swallowed at one time.
However, according to R. Elazar both Rashi and Tosafos would agree that if one would eat a half kezayis and after a slight pause eat another half, it would qualify as an eating בבת אחת to be chayev. The concept of כדי אכילת פרס would have only permitted the pause to be longer between the two half kezaysim. Since we don't apply כדי אכילת פרס, the pause must be very slight - רש"י - שנתרחקו זה מזה מעט.
The Rambam (Ma'achalos Ha'asuros 5:3-4) has a very different approach in this gemara. Firstly, the Rambam understands that the case of חלקו מבחוץ is not when one eats a half kezayis and then another half kezayis as rashi says, nor is it a case where one splits the kezayis in half and puts both halves in his mouth together as Tosafos says. Rather, the case is where he separates the meat from the bone so that it is no longer one unit. The Rambam understands that the chiddush ha'torah which allows you to combine the bones to make up the kezayis only applies when the integrity of the limb is maintained. As soon as one separates the meat from the bone, the bones are no different than bones by other issurim which don't qualify as an eating. Therefore, the case of חלקו מבפנים that you are going to be chayev is when one puts the meat in his mouth with the bone, and only in his mouth do they separate from one another. Since they were placed in his mouth together, they combine.
The Rambam also holds that paskening like R. Elazar wouldn't undermine the assumption of R. Yochanan. R. Elazar who allows one to be chayev even if they would break the eiver into pieces and eat a half kezayis and then another half, would only apply if ultimately he ate a full kezayis of MEAT. Therefore, if he actually ate a full kezayis of meat, he is chayev even though he separated the meat from the bone and ate it little by little (even with slight pause in between) which is the halacha of R. Elazar. But, in order to combine the bones with the meat to fill the shiur of kezayis, he must place the bone in his mouth together with the meat and together they must make up a kezayis - which is the halacha of R. Yochanan. In other words, the Rambam understands that R. Elazar takes issue with the definition of eating בבת אחת that R. Yochanan establishes by relaxing the requirement of putting it in one's mouth together (when there is no need for bones to be considered toward the kezayis), but agrees with the din of when bones can be combined - only when they are together with the meat at the time they are consumed.