Assuming you mean that your 2nd set is in another room, and you want to use the HR10-250 for both a main HD set and a remote TV (via coax), you have two options. First is to keep the existing SD unit, which for the $5/month mirroring fee will give you extra recording potential, and the ability to watch different shows on both set.
Otherwise, your best bet to get a coax output is a nice $35 modulator from RadioShack that also supports MTS Stereo (assuming you've got a relatively nice set in the other room, otherwise the cheaper $20 ch3/4 modulator will do fine), and then whatever remote control solution suits you. Disable 480p whichever HD output you don't use (720p/1080i), and then a quick two-arrows-up on the HR10 remote will switch the output resolution between HD and 480i SD so you can see the show in the other room...
I've posted this info before - search for more info if this isn't clear...
Jeff