db={"Products_type_1":[{"name":"product_1","user":"user_1"},{"name":"product_2","user":"user_2"}],"Products_type_2":[{"name":"product_3","user":"user_1"},{"name":"product_4","user":"user_2"}]}

db.Products_type_1.aggregate([{"$match":{"user":"user_1"}},{$unionWith:{coll:"Products_type_2",pipeline:[{"$match":{"user":"user_1"}}]}}])