I just found a nifty trick worth sharing.
I have (like most of us) multiple mailboxes, most of them hosted in Office 365. Each mailboxes has its own calendar.
I was looking for a way to get my To-Do bar showing the appointments of calendars other than the default one (and it is apparently not supported by Outlook 2016). If you ever find another mechanism, let me know!
I found a simple yet customizable way of achieving this by providing a custom HTML template to display your Today. If it is also interesting to you, have a look at http://outlooktoday.webs.com/