How do sub-weapons affect attack?

So,my main character is a dual wield and I'm currently using 2 npc-made 3rd Anniv Sword VII. The main weapon has 215 atk and is refined to +8 and the sub weapon has 213 atk with no refinements. When I only equipped the main one, ( unequipped the sub one ) my atk was 1160 and when I equipped the sub one as well,it became 1260. In other words,equipping my sub weapon gave me a 100 atk boost. Will the boost be same for any other 1h sword with different atk? How much will the difference be when Equipping a different sub weapon with lower/higher atk? How much atk boost can I get from refining my sub weapon? Or will refining my sub weapon be worthless?

 

All your sub needs is refine, element, stability and good attack.

 

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