R. Moshe Feinstein on How to Give Birchat Kohanim When You Only Have Kohanim (3 Iyar)

שו"ת אגרות משה אורח חיים ב:לא 3 Iyar 5719 (1959)

Hashem tells kohanim to bless the Jewish people with the famous words that start "May the Lord bless you and keep you." That's a blessing from Hashem, through the kohanim, to the rest of the Jewish people. What if a town has only kohanim? R. Feinstein arrives at the theory that ten kohanim sit in the audience while the rest administer the berachah. Why?