You should see a Remote Disc icon under Devices in Finder on your 2012 Mini. Insert your application DVD into the 2009's drive. Go to Sharing and check the DVD or CD Sharing. Go to your 2012 Mini and click on the Remote Disc icon. You should see your 2009 Mini and the DVD that's in its optical drive. Double click the DVD in Remote Disc and away you go!
Note: There is a checkbox on the DVD or CD Sharing screen for "Ask me before allowing others to use my DVD drive". You can uncheck that so you don't have to always ask to use it on the 2012 and reply from the 2009 to accept the request.