I use Google calendar as host and sync the iPhone and iPad
to it automatically so all three are always in sync
I also link Google calendar to Lightening calendar an addon
to Thunderbird which is my email client
so in reality working calendars are Lightening,iPad and iPhone all in sync
Sounds complicated but all works transparently
OR use iCloud
I assume you're not syncing to Exchange, right? If not, there are two (mutually exclusive) options:
Option 1: (Cloud syncing)
On both, go to "settings->iCloud" and turn Calendars "ON". This will sync all your calendars to the cloud and each other. Note, I'm not entirely sure what will happen to your local calendar entries the first time you enable this.
Sync both calenars to iCal (Mac) , Outlook (PC) (or a couple other options) via ITunes. Just go to the "Info" tab of the device in iTunes and select "Sync Calendars" and the appropriate local calendar utility.