Show multiple calendars in overlay view by default

There is an option in each calendar to displayed it as an Overlay. Right click on each calendar and select Overlay:

Overlay can also be set from the view menu

You have to change the default settings that control when the display changes back and forth. You'll find that in Options. Once done, open all the calendars, click the arrow that makes them glue together and they should stay that way.

The main problem is that Outlook doesn't remember your preference to show all the calendars so when you reopen Outlook you have to reselect the ones that aren't showing. To make that easier, put them all in one group along with your default calendar, then you can just click on the group rather than each calendar.