Last weeks parsha (this coming weeks parsha in America), Naso, has the mitzva of ברכת כהנים. An interesting question that comes up is can a non-cohen perform ברכת כהנים? This is not so farfetched, every Friday night fathers bless their children with ברכת כהנים, is that problematic?
The Gemara in Kesubos 24b seems to say that there is an issur עשה for a non-cohen to go up and duchen (see Rashi there who learns pshat that way). However, Tosafos in Shabbos 118b can't find an issur for R' Yosef a non-cohen who went up to duchen. The acharonim work hard to explain this contradiction.
2 of the acharonim disagree with Rashi's pshat in Kesubos and hold that there is no issur. The הפלאה explains as follows. A yisrael has a mitzva to receive the Cohen's beracha, therefore if a Yisrael goes up to duchen he is violating that עשה of not receiving the Cohen's beracha. It comes out that if he is alone (there are no Cohanim), then there is no issur. The Nesivos has a similar idea. He says that there is an issur for a Cohen not to go up and duchen if he is called. If we will make the Cohanim go through all kinds of Yichus checks before letting them duchen, Cohanim will be reluctant to go up and will violate this issur. Again, there is no issur on a Yisrael to go up and duchen.
The consensus of acharonim is that there is an issur for a non-cohen to duchen.
The Mishna Berura at the beginning of סימן קכ"ז, discusses why it should be permitted for a father to bless his sons with Bircas Cohanim. He differentiates based on what your kavana is, when you bless your children your kavana is not the mitzva of Bircas Cohanim. There is a Rama there as well as a Magen Avraham who offer various answers to the above contradiction.
The Gra because of this issue would not give a beracha with 2 hands. he held that the issur was only בנשיאת כפים and therefore without spreading out both hands there is no issur.
I think the minhag is to be מקיל to use 2 hands although some do follow this Gra.
