I know this might not be of help if this isn't a setting you can control, but for one of our stores the splitting takes place on the e-commerce platform side of things. Alcohol goes to shipment one--filled by a different company--and non-alcohol items go to shipment 2, filled by us. So each order comes into ShipStation pre-split. Hopefully that's something you have control over.
Basically for a year either me or a team member manually split the orders (have a whole training video on it). Finally, one of our team members had enough and wrote some code with the shipstation API to do it automatically. Basically takes a list of SKUS that are meant for each warehouse and duplicates the orders to split them. Let me know if you want to hop on zoom and I can show you or email if you'd rather not zoom. May sell it as a service haha (copied this from my other forum response to a similar question)