Dual wielding Beast Barbarian

[ProsecutorGodot]

Do you have Extra Attack? You can just do one swing with your tail and one with your rapier, getting sneak attack with the latter. And once you attack with the rapier, you can attack with another weapon in your offhand like a dagger if you're after TWF. This will give you a second opportunity to sneak attack if you missed with the rapier. I wouldn't bother with Dual Wielder or anything like that.

You would need dual wielder to use the rapier, both weapons need to be light otherwise.