When helping another, it would be just a matter of selecting a character that you are going to provide advantage to. Then, on that character's turn, both characters would perform the same action. In other words, if Lae'zel was going to help Astarion, she would click on the help button and select Advantage, or something like that. Then she could select Astarion to give him advantage and therefore sneak attack. End turn. On Astarion's turn, he does sneak attack via bow. Select Target. Both he and Lae'zel fire a ranged attack. If she has no ranged weapon, they could make it so that she scoops up a rock and throws it. After all, she's only helping him by distracting the foe. She doesn't need to actually shoot or throw something that would hit.
Likewise, when helping in melee, the person who is helping should be shown as swinging their weapon or throwing a punch at the same time as the one who is truly trying to hit.
