yes, easier in that it keps all your combat with one hand. Here, though, are the advantages of involving the second hand:
You will always make a secondary attack when you want to do so, instead of sometimes discovering that your double-click wasn't fast enoughand ending up with a normal attack. or no attack.
It allows you to forget completely what you do with the combat hand - attack or defend, thats the only choice. The other hand makes all the other decisions.
It keeps the choices of how to attack with the 'non-mouse' hand - at the moment, if you prefer a slash to a stab, you maneouvre with the moevvement keys to be in position. If your movement keys are ASDW, and you make Lshift the secondary attack trigger, then it is part of the same thing and all you have to do is hold your little finger on it.
And, if my player has exceptionally high weapon-speed, and wields a very fast weapon, I can get two primary attacks in with a 'slow' double-click. I'd not want this to be impossible because slow doubleclicks were recognised as a secondary attack instead.