According to Rav Zeira, the din of ‘Tamchuyin mechalkin’ (the manner in which the food was prepared) does not apply to produce a leniency. If the manner of preparation determines its type, how can we differentiate between kula and chumra?
Reish Lakish learns from the passuk of ‘Meichataso venislach lo’ that one korban is sufficient for both portions of chelev, although he had discovered the first one before the second. Rashi explains that the passuk teaches that even atonement on part of the sin is sufficient for the entire sin. Why is our case considered a kaparah on only a portion of an aveira?
Normally we can say that we are Machmir because we aren’t completely sure. However, in this case this obviously doesn’t apply, since the nature of the Chumra is to bring another Korban — something we never do unless we are sure it is necessary. Otherwise, iot is Chulin Ba’azara.
So, the answer over here is that we are aware that at the root, they are of one type. When there is less than a Shiur of each we do recognize that he ate one general type of food. When there are two Kezeisim we focus on the fact that he had two different foods, although they are related. It is like two branches in a tree. When they are big enough to be reckoned with on their own, they are two. Otherwise, they are both of one tree.
One Shegaga is one sin. As we find by Ketzira Gorer Ketzira, where Rashi says that it is not even Gorer, just merely one Chet, and Tosafos applies this thinking to Ketzira Gorer Techina.