No, there is no method to assign a context to more than one calendar.
FYI: OmniFocus and iCal syncing has been disabled in Mountain Lion because Apple has decided to deprecate the APIs for iCal and task syncing.
*sigh* time to change my workflow...
Here's the link for the info on iCal and OmniFocus:
Apple has made changes in OS X Mountain Lion that interfere with the iCal Sync feature in OmniFocus. These changes include deprecating the Sync Services API and converting local calendars into iCloud calendars. Depending on your exact calendar setup, it may misbehave in a variety of different ways.
Because of this, the iCal sync option has been removed in the latest update to OmniFocus for Mac. We apologize for the inconvenience this causes folks who were able to put the feature to other uses, but it has far outlived the intended purpose, which was syncing to-dos with pre-iPhone PDAs. We don’t want additional customers building their workflows around this.
Rather than spend more energy keeping this feature limping along, we’re going to continue working on giving OmniFocus a new interface and foundation for features that will benefit all our Mac customers. Thanks for understanding.
This is not related to Sandboxing.
You might try experimenting with syncing to Reminders if you decide to upgrade to Mountain Lion. It is found in the Sync Preferences screen. You can publish "due" tasks as reminders for use in Mountain Lion Reminders app.