If a character is fighting an opponent who is fighting, and a second opponent attacks the character, would the DM rule that the distraction of dealing with the fought opponent makes the number 2 opponent have an advantage in his attack?
I guess it could depend on the nature of the Grapple. Simply grabbing an opponent by the wrist to prevent his slingshot could leave him alert and ready to dodge or dodge, but a more violent fight, which involves a grip with both arms, I guess he would leave it open for a blow from behind and thus at a disadvantage in front of a second opponent.
Even if you have continued with a move to leave your opponent holding Prone, I imagine that you are now kneeling, crouching or distorting your combat stance in that way (at least I cannot imagine that dealing with a prone opponent can stand up) as to give a second opponent the advantage.
I don't see anything in the Rules (I have the Essentials kit Rule Book) that covers this.