One very important thing that everyone has left out, is that as soon as you plug composite cables into the back of the genie and hook into a rf mod, your going to get this nice ugly screen that says YOUR TV OR ITS CABLES ARE NOT HD PRESS AND HOLD EXIT TO ENTER STANDARD DEFINITION MODE. sorry for caps, but it is a big ugly screen lol.
Point is, by mirroring, your will effectively kill the hd signal going to the original tv. Unfortunately there is currently no way to remedy this unless you feed your second tv via component (5 cables) or a second hdmi with a hdmi splitter or switch.
Paying the extra 6bucks a month is probably a simpler solution, but where there is a will there is a way.