He hid the skill to avoid the attention and envy of other players.
Weapon skills that didn’t have clear conditions for appearing are
called extra skills. They were sometimes even called random
conditions. An example would be Klein’s «Katana». But «Katana»
wasn’t all that rare and appeared quite often as long as you kept
training the Curved Sword ...
He didn't want to show that he was strong to everyone and if he showed his dual swords he would have got a lot of publicity.
He didn't want the unnecessary attention, if everyone found out he had something so unique then people would be jealous of him and start stalking. They would start questioning him about where he got the sword from.
He would make a lot ...
I agree with him not using it because its too powerfull it would be unfair for others but thats assuming due to his good character.
And another thing is maybe its because he has another sword made by liz which he prefers to use better when wielding dual sword.
But I disagree with the thought of he doesn't use it much because shinon only retrieved it for ...