Twinned spell allows you to cast a single target spell and target a second target with it. If you have advantage from something that says "you have advantage on your next attack" (like silvery barbs for instance) do you get advantage on both targets? The flavor of Twinned spell is that you are splitting a single target spell to hit a second target - It is a single attack being made which targets two enemies so I assume it fundamentally the concept of advantage would apply to both (not really worried about the RAW just what you think seems correct based on the wording of twinned spell.

Keep in mind there are plenty of things that would apply field advantage like cover and fairy fire, in which obviously both would get the advantage - but im curious wether the use of Twinned spell can be used to stretch a single action advantage a little further.

When twinned spell is applied to a spell attack, you have two distinct attack rolls. If you have advantage on the 'next attack', that is referring to a single attack roll. As such, you must designate one of the two attack rolls as your 'next' one and that gets advantage. The second attack roll does not get advantage

As Davyd says it would affect only the next attack roll, as per the wording of the Silvery Barbs.