Yes to Flanking!

The optional rule on Flanking in the PHB will be implemented in this campaign setting for both the player characters and their enemies. This means that if two player characters are exactly on opposite sides (180 degrees apart) and both are within 5 feet of the creature, both characters will have advantage on all melee weapon and melee spell attack rolls. Note that this also works for the DM's creatures. The intention of this rule is to make movement during combat more strategic.