The damage will increase by 1 point for each unit and they will start inflicting say 20 phys damage + 1 poison damage per attack. The same goes for Isshara's whip which gives +1 fire damage. I tested it and it works. I'm just not sure whether you can do ... um... how to put it... prismatic damage (3+ sources) if you put multiple items though.
I've tested the 2-type damage and it works like a charm
As for the poisoning, I'm not sure. I guess 1 damage doesn't actually give you a big chance to poison the enemy.
I'll see if I can test it with poison dagger and the whip... provided I have that dagger
EDIT: Nah
No dagger whatsoever. Maybe I was thinking of another game. Although it would be uber to have the dagger + the whip. Then the power girl party will become absolutely deadly. Sprites do 1-3 damage, with whip 2-4, with dagger 3-5. The lake fairies will inflict 3-4 damage. And that's before all other bonuses to kick in.