db={"Test":[{_id:"A"},{_id:"B"},{_id:"C"},{_id:"D"},{_id:"E"},],"Dual":[{id_:"dumy"}]}

db.Test.aggregate([{$project:{_id:1}},{$unionWith:{coll:"Dual",pipeline:[{$project:{_id:["D","C"]},},{$unwind:"$_id"}]}},{$group:{_id:"$_id",total:{$sum:1}}},{$match:{total:1}}])