You would have to find 3 adjacent channels not used by the cable system and set the modulator for the middle one. This is difficult because the cable system tend to use all the channels unlike antenna/over-the-air (OTA). Digital cable may use more on the high end.
In OTA you will not have any channels used above 50 or 51, these days for the next couple of years--they were auctioned off. BTW OTA channel 39 [or is 38] is never used because it is reserved for radio astronomy.
Thus, it's not a good idea to try to mix Dish and cable on the same cable. OTA maybe.
-Ken