You can sync BusyCal with multiple Google accounts. For example, if you have a personal gmail account and a business Google Apps account, you can configure BusyCal to sync with both of them.
For each Google account you wish to sync with, select Calendar > Add Google Login, and enter your Google account login details.
Note: This method of syncing with multiple Google accounts requires that you enter the google login and password for each account. This is fine for an individual with multiple google accounts. However, if you want to sync with other Google Calendar users without sharing your personal Google Account password, you should follow the instructions for Syncing calendars with remote users using Google Calendar as a conduit.
Once you've configured BusyCal to sync with multiple Google Accounts, each account will appear in the source list and you can subscribe to the calendars on those Google Accounts by checking them.
In addition to being able to subscribe to calendars hosted on Google, you can also publish calendars hosted in BusyCal to Google. To publish a calendar to Google, control-click on the calendar and select Publish to <google account>. If you are syncing with multiple google accounts, you can publish to any/all of them by selecting the appropriate account name in the menu.