[{"usernames":["userA","userD"]},{"usernames":["userA","userB"]},{"usernames":["userE","userB"]},{"usernames":["userA","userP"]}]

db.collection.aggregate([{$project:{"usernames":{"$cond":{"if":{$and:[{$in:["userA","$usernames"]},{$in:["userB","$usernames"]},]},"then":"$usernames","else":[]}}}},{"$addFields":{size:{"$size":"$usernames"},}},{$match:{size:{$gt:0}}}])