An interesting question is regarding a kohen who is able to say the brocha yet cannot hold up his hands. In this instance we relate the known story of the war against Amalek where Moshe Rabbenu was unable to hold up his hands – כאשר ירים משה את ידיו וגבר ישראל – hence Aharon stood from one side and Chur from the other.
Similarly, when a kohen gets up to perform his mitzvah it would be prudent to help him when he cannot hold his hands up for the duration of the brocha (what is sometimes quite lengthy should the chazzan forget that the kohanim are holding their hands outstretched) – we suggest sending young boys up to aid the kohen – one from each side to hold each hand.
It follows then to have other kohanim behind the said elder kohen so that the two aids will likewise be included in the brocha.
In recognition of Rabbi Asher Zeilingold, Rabbi of Adath Israel Congregation of St. Paul for asking this question.