David:
Normally, you cannot run a DC pre-amp powered antenna and satellite over the same cable for the very reason you've already stated, they both are trying to send DC up the cable to power their respective devices, thus the two power supplies fight each other. The Sony is smart enough to realize this and gives you a SHORT error message. This is a general message, which occurs for a number of short/voltage problems, but bottom line is you've got a problem. Also, it may not clear the error untill you unplug AC power to the Sony. The supplied Terk antenna and splitter, however, is designed for this application by passing the Sony DC power on to the Terk, thus powering them both. The Terk has been designed to run on the varying voltage from Sony, and it won't affect the Terk pre amp gain. I believe your problem stems from trying to still use the Terk power supply as well. It is NOT needed in this application. I also assume you are using a standard 2-way splitter from your bedroom to the other TV for the Terk signal. This is fine as long as it's after the diplexer in the bedroom, on the UHF/VHF antenna output.
Scott:
I assume you have more than two sets, so you can still use the Terk by also purchasing a voltage switch. Most have a UHF/VHF antenna input as well as two inputs, one for each output of the LNB on the satellite dish. In this case, on one of the leads between the dish and the voltage switch, you would insert the Terk diplexer at each end. One would add the signals together at the dish and the other break them apart again just before the voltage switch. The two signals would then be connected to their respective inputs on the voltage switch (Sat to Sat, Terk to UHF/VHF). Now that the two signals are mixed together in the switch, you can send the signal to each room as needed one one cable. If the room has both Satellite and TV receivers, then you need a diplexer to break the two signals apart. This keeps the voltage output from the Satellite receiver from being shorted, which would happen if you just used a regular 2-way splitter. If the room has only a TV, then you still need a diplexer, because there's voltage on the lead from the switch that feeds back from the other Satellite receivers. Just put a special, made for Satellite, 75 ohm terminator (blocks DC, still gives signal 75 ohm load and doesn't short power from Satellite receiver) on the unused output of the diplexer. This keeps the system balanced and noise from entering the signals. Also terminate any unused outputs on the voltage switch with the same special terminator.
Sorry for the long note, but wanted to make it clear. Let me know if you still have questions, with an e-mail. I don't always have time to surf this great site, but do read mail daily. I'll post your question/answer here.
Take care.