db={"collection1":[],"collection2":[{nim:1234,docName:"two"}]}

db.collection1.aggregate([{$match:{nim:1234}},{$set:{from:1}},{$unionWith:{coll:"collection2",pipeline:[{$match:{nim:1234}}]}},{"$group":{"_id":null,"from":{"$addToSet":"$$ROOT.from"},"doc":{"$push":"$$ROOT"}}},{"$match":{from:1}},{"$unwind":"$doc"},{"$project":{_id:"$doc._id",docName:"$doc.docName"}}])