Hello there!
To answer your question you need to know a few things, but don't worry I'm here to explain!
That weapon is a sword. Usually when an Assassin Cross uses a sword it can only be equipped in the main hand (Right hand), but if they are daggers they can be equipped in both the main(right) and off(left) hands.
I assume you're using a Dexterity Sword? That would be the reason why the stats give Crit - 50. There are 2 variants of assassin cross swords, Dex version and Critical version.
Double Attack is a passive skill which only applies to daggers.
In this case, the sword isn't affected due to it being a sword.
If you use sidewinder you'd be able to get double attack on the weapon it is compounded in, in this case the sword, which would mean your damage would now have a chance to be doubled.
E.g. if your damage was 24k on non-sidewinder and would be 16k on sidewinders, double attack would make that 16k into 32k which, for me, is a good deal.
Hope this helps!