Here's a diplexer on the Sadoun site.
Click the
little drawing and see bigger copy of wiring diagram.
Here's a diplexer on the DishStore web site.
Click
the photo, for a close-up view, where you can see the wiring info on the part.
But be advised, you don't get something for nothing.
There will be a reduction in both OTA signal and satellite signal.
If you can still work under those circumstances, then a diplexer might be useful.