On Uber:
It is possible to do this from the tablet if, and only if, there is not yet a delivery person assigned to the order in question.
Make sure there is no delivery person assigned to the order.
Press "Actions".
Click on "Add drivers".
Choose the number of drivers to add.
On Deliveroo :
It does not offer the option of doing this from the tablet, you need to contact Deliveroo support directly.
Call the Deliveroo support number: +44 (0)207 138 9037.
Give them the ID number of the restaurant in question (ID resto).
Ask them to add one or more additional drivers.