Why doesn't the Gemara bring a simple case of one piece of Safek Cheilev, Safek Shuman?
Hi. Sorry for the delay in answering, but better late than never.
I am not at all certain that according to Rebbi Zeira, one will be Chayav an Asham Taluy if he eats one piece of Safek Cheilev, Safek Shuman, and leaves over another.
Perhaps it is only called Efshar le'Varer if the remaining piece is a Vaday, like in the case of one piece of Cheilev, one piece of Shuman, where the Isur is clarified automatically, but not when it remains a Safek which might become clarified.
This seems to be the case, from the fact that the Gemara cites the case of Chiyuv as one piece of Chilev and one piece of Shuman. According to you, why does it not cite the case as two pieces of Safek Cheilev, Safek Shuman?
On the other hand, the Rashash learns the one and a half Kezeisim in the Gemara, as one and a half Kezeisim of Safek Cheilav, Safek Shuman, which is exactly like you. I do not know however, why he learns that way, since it is against the flow of the Sugya, and, as he himself writes, Rashi does not learn like him.
My Chaveiram tell me that this Sha'alah is discussed by the Pri Megadim in Hilchos Shechitah. However, I do not have the time to look there now.