It is possible to connect multiple Google calendars to Sembly (unfortunately, this option isn't available in Outlook).
In the My Calendars section select Google Calendar -> toggle on the calendar you wish to sync with Sembly.
The list shows calendars that are available in your Google account in the My Calendars list. To add more calendars, follow the Google instructions here.
If you wish to add more calendars, give permission "Make changes and manage sharing". Note, some GSuite accounts restrict sharing your calendar to the outside accounts (like on a screen below).
For more information, contact Sembly Support.