This is what I was thinking also. You may find someone (like me and my casual job) where I'm happy to do one or two days maximum but is prepared (like the Christmas rush) to work more hours, knowing it will drop back to a more manageable level once the busy time is over.
You might find more mature ladies (or mothers of younger children) who don't want full time work and they could job share the larger role. This means you have three people all trained, all able to step up if someone is sick or resigns.
These casual staff are good for reception / meet and greet roles but what I really need is a linch pin, a right hand woman to hold it all together. This is the person who knows most about the practice and is on top of billings and debt recovery.