Not all cable boxes that have multiple tuners also have segregated autonomous outputs. Most of the time the extra tuners are used to record one channel while watching another. The Slingbox Pro-HD can allow this, but it is dependant on the use of the internal hybrid analog/digital tuner. Since you can watch the Slingbox Pro-HD's internal tuner, you wouldn't affect the output of an external set-top box. The only caveat to using the Slingbox Pro-HD tuner is that it won't be able to tune in encrypted channels. Since it only supports over the air ATSC and clear QAM signals.