While you may have multiple calendars connected to Existence, you will only have one Primary Calendar.
This is the calendar that new Time Blocks will appear in by default. Upon creation you can always select a different calendar (if it has full access permissions) to create the Time Block in. This option can be found at the bottom of the new Time Block while in Edit mode.
Note: saved, copied or suggested Time Blocks will be created in the calendar they originally were created in, this cannot be adjusted.
Even if you have multiple calendars with Full Access, Existence will only sync the Time Block as an event back to one calendar. The one selected for the Time Block.
Additionally if you're Existence calendar is selected as the calendar to create the Time Block in then the event won't appear on any other connected calendar(s).