SyncCal can copy all events from one calendar into another and keep them synced. I.e. it will ERASE all events in the target calendar and then copy all events from the source calendar into the target calendar, so that the target calendar contains the same events. It can also watch the source calendar for changes and make the same changes in the target calendar (this automatic sync is available as an In-App purchase. The manual sync can be used for free).
Please ensure that you select the correct calendars, because the target calendar will be completely erased. The app has been tested and seems to work fine but you use it on your own risk.