You can Sneak Attack IF you wield a Finesse weapon (such as Dagger or Shortsword) and then either:
(a) you have Advantage on the attack
(b) you do NOT have Disadvantage on the attack AND the opponent in question has the Threatened state, i.e. is under melee attack by an ally
While (a) is indeed occasionally the case, (b) is the far more usual situation.
You CAN Sneak Attack with the offhand attack, and thats the main reason why IMHO one should always dualwield on a pure Rogue - if your main hand misses, you may still get the Sneak Attack on the offhand attack, and Sneak Attack is a nice amount of damage indeed, especially on a pure Rogue.
To my knowledge you cannot sneak attack with the offhand if that attack is unarmed, as it would be the case with Monk bonus action attacks. Either way with 3-4 Rogue levels its "merely" 2d6 extra damage, once per round, so its not THAT much of a loss if it doesnt trigger.
In this build you can get Dualweapon style (or how its called) by trading 1 level of Rogue for 1 level of Fighter; this would lose you a feat though but then Dualweapon basically is a feat. Otherwise the fourth level of Rogue gives absolutely nothing but a feat, so unless you have a feat that you consider important to get, I'd strongly prefer taking the Fighter level.
AFAICS the dualweapon feat does almost nothing in regards to damage and should be avoided - unless you have two non-light weapons you want to pick for their extra effects. Which ironically is most often the case for Wizard and Sorcerer who can then wield two staffs at the same time.
And yes, if you toggle off the offhand attack, you can still pick it explicitly, or you can use any other bonus action available to you.
The most annoying problem of this option is not only that it spends your bonus action on the offhand attack, but that it still does so even if the mainhand attack already killed the opponent in question, completely wasting the bonus action for zip benefit. Thats why by default I always switch it off.
The one occasion when you really want to enable dualweapon is when you do your first attack out of stealth, starting the combat and possibly causing a surprise round. Using dualweapon in this occasion makes sure that you do both your main hand and your offhand attack instantly, while otherwise your bonus action and for the record also all extra attacks get lost.
I would like to add that alternatively you can enable the option that doing such an attack out of stealth will not spent your action. I find this option is extremely strong though because it increases your initial damage output quite enormously, and initial spike damage makes a huge difference in this game.
