Database
db={"first":[{"_id":1,"timestamp":ISODate("2024-02-15")},{"_id":2,"timestamp":ISODate("2024-02-16")}],"second":[{"_id":2,"timestamp":ISODate("2024-02-15")},{"_id":3,"timestamp":ISODate("2024-02-16")}]}
Query
db.second.aggregate([{"$unionWith":{"coll":"first"}},{"$setWindowFields":{"sortBy":{"timestamp":1},"output":{"_id":{"$documentNumber":{}}}}},{"$out":"outcoll1"}])