Here's a site along the same lines.
You can see what happens if you stack at non-optimal distances or get one of your leads reversed/crossed.
How to use the graph is a little hard to follow until you follow the examples. Near the bottom see the two antenna trick.
http://www.hdtvprimer.com/ANTENNAS/ganging.html
I guess there's an indoor antenna description to see first
http://www.hdtvprimer.com/ANTENNAS/silver.html#TAT