In checkout, simply click on the 'Add Another Client' button:
Find the client you wish to add and select them from the clients listed:
You can check it is today's client as the search will show 'booking today' if that is the case!
When you click their name, their booking will automatically be added to the original client's checkout.
You can then checkout as normal. Once checked out, both bookings will turn grey in the calendar, indicating they have been checked out.