According to our experience, ICU is not just a canned application which automates a batched tedious task, but the only method we know of, which guarantees a smooth configuration of both types of Google calendars (personal and shared) on iPhone. Without going into much detail one must appreciate that there is a difference between configuring a shared Calendar and a persona Calendar, and often most people have trouble deploying either one or the other on iPhone.
First of all, connect iPhone to your computer. Then get ICU from Apple's website - there is a version for OSX and another one for Windows. Once installed on your computer, start up the application and create a new configuration profile:
Next you need to enter the configuration details for each of the Google Calendars, similar to the following:
For those wondering, the Cal_ID is the Calendar ID which you can obtain from the Google calendar settings. This looks like an email address.
Finally you should see an Install button in the top right corner of the ICU application:
Click on install and allow some time for this configuration profile to be installed on iPhone.
Now connect iPhone to a nearby wireless network and allow some time for the new Google calendar to appear in your calendars list! Now you can access the calendar and make two-way syncs (you can create appointments from Google or directly from iPhone, and in both ways, synched accordingly to the other mirrored calendar).