I prefer the CalDAV approach so you may find that works for you. These are actually the instructions I created for syncing multiple Google calendars so just adapt them to suit.
First, in the Google Calendar settings (in the browser, at google.com), go to the settings menu, and find the secondary calendar you want to add to your iPhone.
Go to Settings> Calendars> [Click on the secondary calendar you want to add to your iPhone] ... scroll down to the bottom of the window and look for the Calendar ID - it's beside the three colored buttons (XML, iCAL, HTML).
It should look like this: [a string of numbers and letters]@group.calendar.google.com
You need to copy that address (the entire thing, including the @group.calendar.google.com).
Next, you are going to supplement that email address for your regular @gmail.com email address in the long URL. This is the long URL structure (what you will add to the CalDAV address box on the iPhone):
https://www.google.com/calendar/dav/[string of numbers and letters]@group.calendar.google.com/user/
iPhone
Settings -> Mail, Contacts, Calendars -> Add Account -> Other -> Add CalDAV Account
Server:
https://www.google.com:443/calendar/dav/{string of characters}@group.calendar.google.com/user/
Username: {account}@googlemail.com (or gmail.com in most cases)
Password: {password}
Description: {whatever}
Tap 'Next' button at the top.
Doing it this way, using the personal account's credentials, results in the message saying 'verifying calendar', followed by 'calendar verified'.
iCal
iCal Menu -> Preferences -> Accounts -> +
Description: {whatever}
Username: {account}@googlemail.com (or gmail.com in most cases)
Password: {password}
Account URL:
https://www.google.com:443/calendar/dav/{string of characters}@group.calendar.google.com/user/
