quotes a machlokes R' Yochanan and Reish Lakish whether chatzi shiur is an issur d'oraysa or derabbanan. By coincidence, the daf yomi recently discussed the machlokes R' Yochanan and Reish Lakish whether achilah means hana'as garon or hana'as mei'ayim -- is eating all about the mouth or the stomach? Nafka minah: what if a person swallows a chatzi shiur, regurgitates it, and then eats the same chatzi shiur? The mouth has chewed threw a full shiur, but the stomach only got half.
R' Tzvi Pesach Frank (Shu"T Har Tzvi) quotes someone who suggested that these two arguments work l'shitasam. The gemara in Yoma explains that according to R' Yochanan chatzi shiur is assur because it is chazi l'itztarufei, the half shiur has the potential to combine with another half to make a whole issur. R' Yochanan is l'shitaso that when a half shiur is swallowed, regurgitated, and then re-eaten, the second eating combines with the first because hana'as garon is what counts. Reish Lakish looks at hana'as mei'ayim and therefore a half shiur can never recombine with itself to add up to a whole, which is why he holds chatzi shiur is permitted.
The weakness with all this (as R' Tzvi Pesach Frank explains) is that chazi l'itztarufei means a half shiur can be combined with other pieces, not necessarily that it can combine with itself. Nonetheless, it's clever and inyana d'yoma. (I always like hearing l'shitaso arguments because I find it really hard to think of them myself and usually there is some clever twist involved.)