The Great Weapon Master feat is currently a toggled ability in the reactions window and I think Sneak attack for rogues would work well by using the same system. As it is I attempt to sneak attack with asterion every time anyway since it doesn't hurt my damage to try even if I'm not eligible for a sneak attack. I know I can tell whether or not it will happen by checking my advantages and disadvantages on my hit % but I just do it anyway. The added benefit of making it a toggle would allow us to add sneak attack damage to the first attack that lands. In 5e you can add your sneak attack damage to your bonus action offhand attack in the event your main-hand strike missed.
