JSON_MERGE

JSON_MERGE(json_doc, json_doc[, json_doc] ...)

SET @json1 = '[1, 2]';
SET @json2 = '[3, 4]';

SELECT JSON_MERGE(@json1,@json2);
+---------------------------+
| JSON_MERGE(@json1,@json2) |
+---------------------------+
| [1, 2, 3, 4]              |
+---------------------------+