Although any appointments, reminders or other information entered into your iPhone's Calendar application is backed up during a sync with iTunes, this data isn't automatically synced between devices just because they're on the same iTunes account. iTunes keeps data such as calendar entries and address books separate when syncing multiple devices. By setting up multiple devices to share the same online Google calendar, all dates and appointments are synced whenever either iPhone launches its Calendar application.
Tap the Settings icon on the first iPhone. Open the "Mail, Contacts, Calendars" page.
Press the "Add Account" button and select "Microsoft Exchange" from the list of available services.
Enter the email address of your Google account into the "Email" and "Username" boxes. Enter your password into the "Password" box. Press the "Next" button to continue.
Press the "Cancel" button when the "Unable to Verify Certificate" box appears. This isn't an error, but a necessary part of the set-up procedure.
Type "m.google.com" into the new "Server" box that appears on the account set-up page. Press the "Next" button again. Google will successfully verify the account.
Slide the "Calendars" switch to "On." The iPhone's Calendar application will now be automatically synchronised with the Google calendar whenever the app is launched.
Perform the same account set-up procedure on the second iPhone, entering the username and password of the same Google account. The Google calendar will now act as an intermediary between the two iPhones, and synchronise any changes made on either device.
If you don't want to share an existing Google account with a second user, set up a new one to sync the calendars between your iPhones. You can also use this method to sync address books between iPhones by sliding the "Contacts" switch to "On" on the "Exchange" page. You can add more devices to the same Google account using this method, and their Calendar apps will also be synchronised. You can log in to the Google account on a computer and add data to the calendar, which will then be synced to both iPhones next time either one launches the Calendar app.