To do this, you need to combine the src
strings from the embed codes. So if you want to display these two calendars:
<iframe src="https://calendar.google.com/calendar/embed?src=555555555555555555555%40group.calendar.google.com" style="border: 0" width="800" height="600" frameborder="0" scrolling="no"></iframe>
<iframe src="https://calendar.google.com/calendar/embed?src=FAKEEMAILADDRESS%40gmail.com" style="border: 0" width="800" height="600" frameborder="0" scrolling="no"></iframe>
You would combine them like this, using an ampersand:
<iframe src="https://calendar.google.com/calendar/embed?src=555555555555555555555%40group.calendar.google.com&src=FAKEEMAILADDRESS%40gmail.com" style="border: 0" width="800" height="600" frameborder="0" scrolling="no"></iframe>