I have the tablet hooked up via USB to a receipt printer and a further USB printer in kitchen. 

I want to allow waiters to use mobile devices to input orders.  The tablet would act as master and generate the order printers through its connected printers. 

This doesn't seem possible as each device requires a printer connection. 

Has anyone found a way to use phones as a slave device rather than its own independent POS device. 

Hello.
Unfortunately, there is no such mode in Loyverse POS as slave or master devices.
I would recommend using Ethernet receipt printers to print kitchen orders. It is more convenient to have mobile devices with Loyverse POS, not tight by a cord to one place.