לזכות רפואת אבי מורי ר' יצחק יונה בן חנה ור' מרדכי דוד בן קריינא בתוך שח"י
If someone eats half a kezayis worth of mezonos and half a kezayis worth of shehakol, he makes a borei nefashos.
The Magen Avraham [210-1] writes that if one eats a half a kezais worth of shehakol and decides to stop eating and then decides to have another half a kezayis - he makes a new shehakol but the two halves still combine and he also makes a borei nefashos.
The Aveni Nezer [או"ח ל"ח] writes that if he eats half a kezayis shehakol and half a kezayis mezonos and then decides to eat another half a kezayis mezonos - he should make an al hamichya. This is despite the fact that he was originally obligated in borei nefashos. The Avnei Nezer finds a proof for this in ...dinei pigul! The gemara [Zvachim 31a] says:
איתמר חצי זית חוץ לזמנו חצי זית חוץ למקומו וחצי זית חוץ לזמנו אמר רבא ויקץ כישן הפיגול.
It was stated: If one had intent to eat half an olive-bulk beyond its designated time, half an olive-bulk outside its designated area, and another half an olive-bulk beyond its designated time, Rava says: In such a case, the piggul: “Awaked as one asleep” (Psalms 78:65), i.e., since one had intent to consume two halves of an olive-bulk beyond their designated time, these intentions are joined together.
Just like we combine together the two מחשבות פיגול - we combine to two half shiurim of mezonos.
But this is all of he didn't make a borei nefashos on the first half kezayis mezonos and half kezayis shehakol, but if already made a borei nefashos, it would seem that the borei nefashos is rendered unnecessary and he would have to make an al hamichya on the second half kezayis of mezonos. That was the opinion of Rav Genechovsky ztz"l. Rav Dov Landa Shlita wasn't so sure...
