db={"collection_1":[{fieldX:"x",fieldY:"y",date:"2022-01-15 12:00",condition:{enabled:true,condition:"abc",}}],"collection_2":[{fieldZ:"z",date:"2022-01-15 15:25",condition:{enabled:false,condition:"bce",}}]}

db.collection_1.aggregate([{"$unionWith":{"coll":"collection_2"}},{"$sort":{"date":-1}},{"$limit":10}])