Auto-print packing slips when using different format/printer from shipping label

[RunnersPlus]

We use a 4"x6" format for shipping labels and RS labels but 8.5"x11" for the packing slip. When we complete an order and print the shipping labels, we can't automatically print the print the packing slip, too, because it comes from a different printer. 

I'd like to suggest ShipStation adds that as a capability.