Can i use two different runeword swords in dual wield and will they stack?

i understand that two weapons apply their buff seperately when hey hit. but why do some buff overide others? i cant understand why when i have a weapon with crushin blow and open wounds in one hand and a separate weapon with deadly in the other i dont have all three on the stat sheet.

also if i have two exact runeword swords do they need to be the same base to work together? will they stack if they are the same ?

I apologize in advance if these are stupid questions but i dont seem to be getting it and yes im new to the game.