Hi Matthew Laurance ,
please try to put the DML statements inside the "Parallel execution block " and check.
For an example
CREATE PROCEDURE ParallelUpdate AS ,
BEGIN ,
BEGIN PARALLEL EXECUTION,
UPDATE CTAB1 SET A = A + 1;,
UPDATE CTAB2 SET A = A + 1;,
UPDATE CTAB3 SET A = A + 1;,
UPDATE CTAB4 SET A = A + 1;,
UPDATE CTAB5 SET A = A + 1;,
END;,
END;,
For more information please check the below link:
http://help.sap.com/hana/sap_hana_sql_script_reference_en.pdf --> section 7.13(Explicit Parallel Execution for DML)