There are a couple of ways to assign more than 1 workout a day to your client's training calendar.
Assign from workout library
Simply click on the + button on a date, then add a workout from the workout library or create a brand new workout.
Copy and Paste a workout from another date
Similar to copying and pasting to an empty date, you can also copy and then paste a workout to a date with existing workout(s).
Tips & Tools:
Rearrange order of the workouts
Click on the dropdown (...) icon next to the workout title, you can then click on Move up or Move down to change the order of the workouts
Drag and Drop exercise between workouts
In the example below, click on + 1 more exercise or + 7 more exercises to open the expandable popup, you can then use drag and drop on any exercises.