Sync calender on multiple iPhones?

[lPHONE]

My girlfriend and I are both really busy people and I to wanted to know if there was a way that we could both stay in sync with our schedules using our iPhones so that we would both be using the same calender and it would update when either one of us made a change?

 

[Darwin]

The best way I see is to setup a CalDAV account, this can be done for something like a Google Calendar. By using the same login for both phones you should be able to be kept in sync and to make and edit events from either phone.