Sorry, I misspoke. For parallel workflow to work with the rule-based workflow template, you should look into this document: Set up Parallel Workflow Tasks with BRF+ (BADI USMD_SSW_PARA_RESULT_HANDLER)
However, for MDG-S, there is already another workflow template that you should be able to configure. It does parallel approval out of the box (only for MDG-S). See the documents above. You can get to it in MDGIMG under the Supplier Node.