With the new HD GUI, you can't have an HD GUI on one TV and then an SD GUI on a second TV driven from the same receiver. The DVR does not have enough processing power to handle two different GUIs at the same time.
You can get around this by using a component-to-composite converter ($40 from Monoprice). Connect this to the component outputs of the DVR, then connect your existing modulator to the converter. The DVR will think that both TVs are HD and so will output the HD GUI to both. People have reported this works fie, although some of the text is slightly difficult to read.
If you are already using the component output, there are HDMI to composite converters also.