How can I show Growl notifications on both displays?
Currently there is no way to accomplish this. The Growl mailing lists have discussed allowing Growl to always place the notification on the display that you have a window focused in.

For example, you have 3 displays, oriented in a horizontal line, and you're doing some file management on the far-left display and web browsing on the far right. If Finder has focus, the notification will be shown on the far-left display, if Safari/Chrome/Firefox/$BROWSER has focus, the notification will be shown on the far-right display.

But this feature has not yet been implemented, so you are limited to explicitly configuring Growl to send notifications to a single display.

This feature has been written up on their Google Code Project page, so you can star the issue in order to receive further notifications.

Note that you can star it on an issue detail page by clicking the star outline to the left of the title "Issue 122: [...]", or at the bottom of the page just above the comment box. After clicking the star in either location, it will turn yellow and have a thicker blue outline, that means you've subscribed, and in essence, "voted" for the feature.