So I am thinking about this as a strategy. I have a mutli-class warlock rogue and the character has a whip he got with his background, he is proficient in it as a hexblade.
Here is the idea - whip in one hand, short sword in the other. Go in and cast booming blade sneak attack with the shortsword, then disengage as a bonus action and move 10 feet away (still in whip reach). Now the fighter goes in and attacks the same guy from 5 feet. On his turn bad guy can attack the fighter, causing a whip opportunity attack (with another sneak attack) or move to attack me taking booming blade damage and possibly causing an opportunity attack from the fighter.
Also the way I read the rules, I would get dexterity damage bonus for both attacks (action and reaction) since this is not really dual wielding because he is not making an attack as a bonus action.
Here is the idea - whip in one hand, short sword in the other. Go in and cast booming blade sneak attack with the shortsword, then disengage as a bonus action and move 10 feet away (still in whip reach). Now the fighter goes in and attacks the same guy from 5 feet. On his turn bad guy can attack the fighter, causing a whip opportunity attack (with another sneak attack) or move to attack me taking booming blade damage and possibly causing an opportunity attack from the fighter.
Also the way I read the rules, I would get dexterity damage bonus for both attacks (action and reaction) since this is not really dual wielding because he is not making an attack as a bonus action.