[...] Both should be able to inflict sneak attack [...]
AND, you shouldn't be able to use an off-hand attack if you didn't also used an attack action at the same turn. Otherwise it means that characters with off-hand get to attack with a bonus action.
a) The actual rules do not allow two sneak attack per turn, for obvious reasons of game balance. Will not argue for nothing on that point. It is a hardcore official rule, which DMs pretty much never change.
b) Official rule reading (by many for years) is that YOU CAN attack only with your Bonus Action offhand (but do not add Ability modifier unless you have a special feature exception). This logic does not apply to Monk's extra unarmed attack for which you must first do a normal Action attack (Monks are not in the game, but just saying for your D&D knowledge).
I am not asking to make sneak attack twice. I am asking to chose if I want to use my sneak attack on my off-hand or my main-hand.
About only attacking with off-hand. Those are the 5e rules:
"When you take the
Attack Action and Attack with a light melee weapon that you're holding in one hand, you can use a Bonus Action to Attack with a different light melee weapon that you're holding in the other hand. You don't add your ability modifier to the damage of the bonus Attack, unless that modifier is negative."
If you don't use your action to attack, you can't attack. It doesn't matter bonus action or not.
