To copy an appointment, press CTRL, select the appointment, and drag.
What should you do then, if you need a reminder for an appointment in an alternate calendar? Suppose, in this example, that on Thursdays you drive the soccer carpool. Many people depend on you, and you want to be sure you never forget.
You would copy the appointment from your soccer calendar to your main calendar to have it in both schedules: one where you look for it, one that reminds you. To copy an appointment (as opposed to simply moving it), you would press CTRL and then drag it to the other calendar.
With the appointment in both schedules, you would not only get the reminder, you would also have that time blocked out in your work schedule. Your colleagues would not request meetings or appointments with you on Thursdays between 3:30 and 5:00 pm.
The important thing is to do what works best for you, for each appointment. With side-by-side main and alternate calendars, you can keep time in its place. And why stop at two?