Directv for cable, Cox Internet... One cable jack.

There is no way to split the cable line running into that jack because they are two completely different services. Cox will have to run another line and install another jack somewhere, choose a central location of the house.

There is no way to do it because DirecTV has all of their channels scattered throughout the frequencies traveling through the coax. They operate between 250 MHz and 2150 MHz.

You may be able do what you're wanting to do with DISH, but you won't be able to use a preamplifier with your antenna if you're currently using one. DISH pushes its signal between 950 and 2150. The problem you're going to run into is you're going to need to use their wireless boxes hooked up via the ethernet or through their wireless gateway in order to avoid using the 650 to 875 MHz backfeeds which is what wired Joeys use.

You will need to know what frequencies Cox is using for its upstream and downstream carriers. Anything below 40 MHz won't work, but there is a crap ton of noise in that area. A lot of cable companies don't use these frequencies, yet some do.

Let's say Cox's upstream and downstream carriers fall within the frequencies I mentioned. You would be able to do it with 2 diplexers. Cox would tie into the UHF/VHF feed, DISH would tie into the SAT feed and the line running into the house would be the IN/Out. Then, you come off the outlet with a short jumper tied to the IN/Out of the diplexer behind the tv, run your Sat side to the receiver and the UHF/VHF to the modem. Of course, that's all in a perfect world if the frequencies are right.

Your best bet is to run a separate line.