Like many people, I had to do this when setting up a Google Apps account - I needed my personal calendar and Apps calendar to both show up on my phone.
You often cannot just add another Google account to an Android. You typically get a "Can't establish a reliable data connection to the server", which is complete crap. Essentially, you are stuck, unable to add another Google account to your mobile - dumb, but true.
However, if you follow the instructions so helpfully provided by Marc Fonteijn, you can get it working. No import/export required. Basically, you share the Calendars between both accounts, and give both sides the ability to edit events on both calendars.
If you run into the bug where "Share all information, and outsiders can change calendars" is not shown as an option: Set up the share email accounts anyway. Shut down all your Firefox windows, restart Firefox, and clear everything in your history. Then sign back in to your Apps Dashboard and try again. "Share all info" should then appear as an option on the previously set up email accounts.
Don't export the info from your first calendar into your second. You'll end up with duplicate events all over the place.
Now if they could only do this with Contacts...