To add a second google calendar on an iPhone where you already set up your first google calendar just visit this url from the browser (safari) on your iphone: http://www.google.com/calendar/iphoneselect

As crazy and esoteric it may seem, syncing your Apple AddressBook to Google really only works when your Mac is synchronized with an iPod or an iPhone. Just plugging such a device and syncing with iTunes triggers the sync to Google. You can also edit a file on your Mac instead to get the same result, as describe on Lifehacker.com:
Change the "Family ID." value to 10001 in ~/Library/Preferences/com.apple.iPod.plist

Thank you Gina Trappani and Josua Ferris!

Google offers an iPhone Google Calendar App to synchronize Google calendars and introduces CalDAV sync for iCal and Sunbird.

See here: http://www.google.com/support/calendar/bin/answer.py?answer=99355

and here: http://www.google.com/mobile/apple/sync.html