Not within the same calendar.
A single calendar in AnnuCal can only contain one event on a specific day. If that day is already occupied in the same calendar, you cannot add another event to that calendar on the same date.
If you want to show different things happening on the same day, you can use separate calendars. This keeps each calendar as a clear planning layer while still making overlaps across your yearly plan visible.
This structure is intentional because AnnuCal is designed for long-term planning, not for stacking multiple daily tasks inside one calendar.