You can take the Ready action after you use a bonus action, sure. What really matters though is how you get your bonus action. If you just have a feature that lets you take your bonus action without any conditions then you can go ahead and do that. But some bonus actions have conditions that must be met in order for you to take them, like the one from wielding two weapons, as I will detail below.
Taking the Ready action does not count as a separate turn. What happens is, you decide on a triggering event, something you can perceive that you will react to in the future before your next turn. Then you pick the way you want to react to it. For example, you can take the Ready action to respond to an enemy approaching your companion and respond by throwing your dagger at him. Note: In order to actually respond to the trigger you described, you must use your reaction for the turn.
Two Weapon Fighting(PHB Page 195):
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.
So from here we can see that, in order to get your off-hand attack with your bonus action, you have to have already taken the Attack action. This eats up your action for the turn, so you will not be able to take the Ready action like this, unfortunately.
What this ultimately means is that you wouldn't have a way to throw a dagger as a bonus action (unless you found a way in-game, like if your DM gave you a magic item). If you could do it though, you would be able to make both attacks with Sneak Attack, as RogueBurger points out in his answer with a reference to a Jeremy Crawford tweet.