Production promotions can be stacked with the following configuration and updates in database. I would strongly recommend testing all different kind of custom and out of box promotions.
Step1 disable the policy 'Product: Any order item can only participate in one promotion' from px_policy
Disabling the policy lets multiple product promotions be evaluated, otherwise just the one with the highest priority goes in (or in the case of equal priorities, whichever happened to be evaluated first.)
update px_policy set status=1 where px_policy_id in (select px_policy_id from px_policy where name like 'Product: Any order item can only participate in one promotion')
update px_policy set xmlparam=replace(xmlparam,'
Step2: Modify WCSPromotionEnginConfig.xml
Modifying the XML lets promotions stack, instead of just evaluating all of them, and picking the best one.
In the WCSPromotionEngineConfig.xml, you have to disable the behavior
Step3: Update all promtions from accelerator. If you want this to affect your exisiting promotions.
you have to update all the promotions ( don't need to change anything, but need to resave the promotion in order to get promotion XML updated). After that, all the promotions for the same product can be combined.