That's a feature used to reskin weapons, you can put one weapon in first and another of the same type (hammer/dagger/sword) in the second slot and the resulting weapons will have the stats of the first and the 'skin' (the look) of the second.
You should be able to put it into the forge and press the little red x on the bottom right of the gui
That'll unforge them and gives you back both weapons (since 1.6), I've never combined two of the same weapon though so hope it doesn't somehow trick the game into thinking it's one