This issue has plagued Growl for ages. And ML's track record with multiple monitor support isn't great.
But it's worth asking: can I display notification center's badges or alerts on multiple displays, or on the non-primary display? Plist tricks?
In Growl perferences, under Displays you have a screen dropdown to choose with screen you want the notifications to appear on. The screenshot is from Growl 2.0.1.