If a practitioner will be unavailable for a set amount of time, see this guide to add unavailable times to their diary. You can then make appointments for another practitioner in the diary (e.g. to cover the unavailable practitioner) in a time slot that the practitioner would normally be unavailable:
Please note: This may affect Online Bookings. Please consider removing relevant period(s) e.g. evenings from the online booking slot under Settings, so they don't get booked. If a slot does get booked for a significant time ahead, you can always decline it.