Huh?! I didn't know it was possible to create an embedded web calendar with a multiple calendar display options!
Turns out this is not a new feature, I found a 2008 article on it. The UI is seriously weird, which is why I've somehow blocked it out.
When you customize the Google code for embedded calendars the customization screen lists, on bottom left somewhat out of sight, all your Google Calendars. You can check the box to include multiple calendars.
What's weird about that? Well, as far as I know, the only way to get to this UI is through the calendar setting control for a single calendar. So you view settings for a single calendar, then click customize embedded code, and you can share multiple. That's a hack.
Once I knew how to do it I could find the official documentation:
... To display events from multiple calendars in the same embedded calendar, simply follow the instructions above to access the Google Embeddable Calendar Helper. Then, select the checkbox next to each calendar you wish to display, in the section Calendars to Display...
Am I the only geek who didn't know this was possible?