By default Appointment Reminder only sends to the first mobile and email address we find in the appointment. This is normally sufficient for most types of businesses.
To turn on sending to "multiple" numbers then please go to Settings > Edit Calendars > Edit and click Edit on the calendar you want to make this change for.
Find the drop down option called "Sync Multiple Numbers" and enable.
Please note: This article relates to Google Calendar. If you are using a different calendar this article might not be applicable.