If you want to do a lot of damage, try to polymorph into one of the humanoid monsters who can use two-weapon combat. It so happens (at least in vanilla Nethack) that these monsters sort-of emulate two-weapon combat by having two attacks, and this is cumulative with actual two-weapon combat. So if you are polymorphed into a dwarf king, and are two-weaponing, then you actually get to hit monsters four times per round.
I once did that with a Knight, dual-wielding +7 Greyswandir and a +7 silver saber, and polymorphed as an Aleax (prolonged polymorph with an amulet of unchanging; I ended the game that way). Four boosted silver saber hits per attack, two of which being Greyswandir's, hence double damage. Plus 4d20 damage against demons, for the silver. Fun fact: I met Demogorgon, and killed it in one turn; it (barely) survived the four saber hits: what finished it was the extra Aleax kick attack (with my +5 boots).
This looks like a bug, actually. But it really works that way in Nethack-3.4.3.