Calendar has now added the ability to select secondary calendars as the calendars that events are booked on and not just primary calendars. The time slots will still look at all connected calendars for availability but it can now write to secondary calendars and not just primary calendars.
Doing this is simple as it is done in the same place where you would select which primary calendar you wanted to use only now it also shows your secondary calendars as options.
As you can see in the screenshot below under the " People and Places" tab of editing or creating your time slots you can now select either primary or secondary calendars to be the calendar that the events will be booked on. In this example you can see where the "Family" calendar is a secondary calendar but still an option.