לכבוד ידידי אהובי החתן ר' יהודה יעקב בן דינה חאשע וכלתו המיוחדת חנה רחל
There is often great confusion at a sheva brachos when it comes time to bentch. How many cups are filled when, what brachos are made on each and how do you mix them?
So here is the story. The Shulchan Aruch [62/9] says that one should bring two cups and the mezamen bentches on one and six brachos are made on a second cup and then the mezamen makes borei pri hagoffen on the first and drinks. He then cites an opinion that all of the brachos can be made on one cup. The Rema rules like the first opinion that two cups are necessary. The reason for this is that we have a rule [Pesachim 102, Sotah 8, Brachos 49] אין עושים מצוות חבילות חבילות - "We don't do mitzvos in bunches" which is why we don't make a bracha with two subjects at the end [Blessed is Hashem who builds Jerusalem and saves the Jewish people], we don't make both kiddush and bentch on the same cup [see the gemara in Pesachim] and we don't purify two metzoras at once or give to sotahs to drink at once. In the same vein, we shouldn't both bentch and make sheva brachos on the same cup of wine. The reason is that it shows that the mitzvos are a pain for us so we try get rid of them easily [Rashi].
The custom is then to take the two cups and mix them together into two separate cups and the chosson and kallah are each given a cup from which they drink.
There is a oft-cited opinioin of the Brisker Rov who learns from the Rambam that the sheva brachos are an extension of bentching. This being the case the second cup on which the sheva brachos are made should be poured before bentching so that bentching is being said on both cups. According to this opinion, if one didn't eat he should not make one of the sheva brachos. If the sheva brachos are an extension of bentching and this person didn't eat, he has no right to make one of the sheva brachos.
Got it? Great. So next time it comes up, you will be the mayveen. If you are still single and there are girls there, add the idea of the Brisker Rov, thus impressing them with your scholarship and ensuring yourself a wife [or at least a date or two]. If you want more lomdus on this - I can give it to you. Anything to help a Yid get married....:-)