BusyCal can sync with both iCloud and Google Calendar simultaneously.
If you have calendars hosted on both iCloud and Google, you can configure BusyCal and the iPhone to subscribe to both services.
In this example, there are some calendars hosted on Google and some calendars hosted on iCloud. BusyCal is subscribed to the calendars on both services.
To sync BusyCal with both iCloud and Google, see:
Calendars hosted on Google cannot be published to iCloud, but calendars hosted on iCloud can be published to Google, as described below. Although republishing an iCloud calendar to Google is possible, it is not recommended. Ideally, you should only subscribe to calendars hosted on each service, as described above.
If you are subscribed to an iCloud calendar with BusyCal, you can republish that calendar to Google.
To Publish an iCloud calendar to Google, control-click on the iCloud calendar and choose Publish to google.
Note: Your default Google Calendar is hosted on Google. Since calendars hosted on Google cannot be published to iCloud, it is not possible to sync your default Google Calendar with iCloud.