I've been hunting through the source books and the forums looking for the specific language on how the optional flanking rule mechanic is applied during an encounter. I have looked in the right place yet. Is it sufficient for two allies to double team a single opponent to gain advantage from flanking by standing shoulder to shoulder, or does the optional flanking rule require them to be positioned so that one of the two allies flanking a single opponent is on either side?
My general rule of thumb is that the two attackers need to be spread far enough apart to legitimately divide the defender's concentration to get advantage. Generally this is achieved by being on opposite side of the creature as it now has to spit its attention in opposite directions. Having two attackers next to one another or even adjacent to each-other doesn't really strain your defensive options much more than usual.
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"The mongoose blew out its candle and was asleep in bed before the room went dark." —Llanowar fable
Advantage equates to a +5 bonus when you do the math, and a lot of people feel that it is too much, so the go to is a +2 bonus.
Not my cup of tea...personally I find that being tactical with positioning should confer a better bonus, and granting advantage makes it to where mooks become more of a threat. A static +2 is good for the party, but not as great for the DM because encounters will be shaped with the +2 bonus in mind both in what sort of AC's you choose for the monsters to have, and in the bonuses to hit those monsters have. 5 Kobald's that can get advantage and force you to move out of a threatened space for OA's is much stronger than a static +2 to hit, meaning that later on you're less likely to use those tactical types of encounters at later levels.
EDIT: Kobald's were a bad example, but you get the point.
I prefer a slightly more complicated flanking rule. If a creature can get all of its melee attackers in less than 180 degrees, it is not flanked, if it cannot, it is. If a creature is flanked, all melee attackers get advantage since it's having to deal with attacks from so many directions.
I started using it because it bothered me that two people can flank but three cannot.