On emails, you trigger manually - like confirmations, accept/decline, and cancellation notices - you can enter another email address on the 'To' line separated by a comma. But unfortunately, there's not currently a way to send all automated messages for a client to multiple email addresses.
The one exception is the reminder. On Client > Details, you'll notice a field called "Email Cc" at the bottom. This lets another user schedule appointments on behalf of the client. To enable this, create a new record with its own email address, and then make that record the "parent." Then the "parent" will also get appointment reminders and be able to see upcoming appointments.
You can add as many Email CC options to a client record as needed.