D&D 5E Advantage when two allies are in melee with the same enemy?

I was going to try to add in more stuff like flanking before dropping the idea as it didn't really add anything to the game. Miniatures are just for location reference and I think the game plays great without all the miniatures game stuff of the last two editions. Advantage/Disadvantage would be something to think of but it would come up too often and require reworking some monsters.

Thanks for the feedback, gang! I obviously was confused, and I think [MENTION=907]Staffan[/MENTION] and [MENTION=12731]CapnZapp[/MENTION] were exactly correct: there was a rogue in the first party I DM'd for 5E, and I think I mistakenly translated the rules for sneak attack onto the rules for advantage. I appreciate the input.