This may be what you're looking for:
I am pretty sure this is the way Dish had mine hooked up. IIRC, my 722 actually used a single triplexer that took the place of separate diplexer and separator at the tuner. Mine was a bit less straight forward since every RG6 is a homerun to a wiring closet, but the circuit is the same. The key for you will be to find where the satellite comes in from outside. If the satellite comes downstairs, terminates in the left-most diplexer, and then runs upstairs to the non HD TV, you should be good for cable runs.
If the satellite terminates into a diplexer in some third space- like on the outside of your house- then your two TV's are connected by a cable that doesn't run directly between the two TV's. This will complicate your initiative.
It can be confusing when you're looking at splitters because you might be seeing the duplex one- a Diplexer- or the separator plus- which is really just a one way split of incoming satellite signal. The tri-plexer works the same way, it's just one unit. The key to getting your head wrapped around this is understanding that the RG6 cable between diplexers has bandwidth to send data in both directions-up and down, to and fro. Said another way, multiple, physical, one way (up.down-to.fro) links are not necessary directly between 222k receiver and the 2nd TV. The signal leaves the 222k, hits the diplexer, and finds it's way back upstairs alongside the incoming satellite signal.
The 722k I used has a slot in the back for an OTA tuner module. I actually still have a module (and a 722k) that I am not using. The OTA tuner allows for any connected TV to use OTA channels.