Multi-target Attacking and Differing Levels of Advantage

How does multi-target attacking work when the attacker has different levels of advantage on the targets?

For example, consider a sword and shield fighter with the feat Multi-Target Attack Specialist II. Attacking two targets is normally at disadvantage 2, but his feat negates 2 levels of disadvantage. This fighter makes a damaging melee attack that targets two guards. One of the guards is prone. The fighter has advantage 1 against the prone guard, and no advantage or disadvantage against the other guard. How does the fighter resolve his attack roll against the two guards?

This is a known bug with Prone, a fix is in the works. The new version will give a static +2 Guard instead of advantage and -2 instead of disadvantage.

What is the solution for other situations where there are differing levels of advantage, such as attacking one foe who is surprised and one who is not surprised, or attacking one foe who is flanked and one foe who is not flanked?

These situational advantages are assigned by the GM. Should he not assign advantage if it doesn’t apply to both targets?

Yeah, that’s the right way to handle it for the rules as written; the GM decides, usually erring on the side of no advantage. It’s hard to take full advantage of a flank/surprise if you’re also targeting someone who isn’t.

If that’s not satisfactory for you, I’ve heard a suggestion before to roll the dice normally, for one target and then roll the advantage dice separately. That can be tricky to keep track of though.