What is your definition of "split orders"?
Are you talking about fulfilling different items from different locations, or something like deleting items from location A from the order and creating a new order with those deleted items?
I'm wondering if the Locations feature could do the trick for you here! Locations will let you have stock in multiple locations and can be used with POS, online store. If one location runs out of stock for a product, it will automatically fulfill the order from the other location if it is in stock there. The number of locations you can have does depend on which plan you're on:
You can find out more about locations from the Shopify help guide about it here. Do you think this is what you're looking to do? If not, can you elaborate a little more on what you want to do exactly.