First of all, if you plan to have both TVs operating at the same time, that is theft of services. With the legalities out of the way, if you have your TV in the living room connected to cable but like to watch TV in bed or in your den, switching the signal to another TV when the primary TV is turned off is permissible.
All you need is a splitter (http://www.radioshack.com/category.asp?catalog_name=CTLG&category_name=CTLG_007_002_001_006&Page=1) placed between the box and the TV. However, you would have to control the channel on the 2nd TV through the box on the 1st TV, which means both TVs would play the same channel if both were turned on.
You could also try placing the splitter before the box. Depending on how much of your cable signal is scrambled, you might be able to get your broadcast channels plus a few more on the 2nd TV.