How can I use one coax for both OTA and genie 2 server

Can't be done they have to be separate and you can't connect an OTA antenna to a DTV receiver without a long dropped and no longer supported AM21 or LCC.

The coax provides 13v/18v or 21v to a SWM LNB to power the transponders.

shannon02 is correct. You cannot share off-air and satellite signals on the same coax cable. 

I'm not sure about the Homerun device. I have a Tablo DVR that connects to my home network and is connected near my off-air antenna with no need to share coax cables. Then I can view its channels and recordings on any device in my house. 

@litzdog911

The Homerun device is similar to yours but it has an ATSC 3.0 tuner. I just don't have an easy way to get a new coax to the outside for an OTA antenna. Maybe someday.

Would be nice if DirecTV could have some integration for an outside antenna. Unfortunately as mentioned AM21 discontinued. The LCC never had an official launch, just in beta testing (seemingly indefinitely) and only available for a limited time during a huge carriage dispute years ago. Plus the LCC wasn't supported on the Genie-2 (HS17), another of that tower's shortcomings.

But with AT&T having sold of their part in ownership perhaps one day we shall hear about equipment changes that will be better for the customer and not an easier one-size-fits-all (and you must like it) that the company encourages too heavily.

There is a possible solution. DirecTV swm channels are in the 950 to 1950 MHz range. MoCa (the communication between receivers ) is in the 475 to 625 megahertz range. Most OTA broadcasts are also in that 475 to 625 MHz range. The conflict is with Moca not the swm channels. So if you take the diplexed OTA signal off the cable before the DirecTV splitter you should be good.

What this would look like then is you have a diplexer outside combining the satellite and OTA signal. The second diplexer is inside before your DirecTV splitter. Put a DirecTV band stop filter between the diplexer and the splitter to block the MoCa signal. Run a coax from the vhf/uhf port on the diplexer to your Homerun. My understanding is that the Homerun is a network device so it doesn't need to hook up to your TV, and could go in the basement or wherever your coax can get to as long as you can hook it up to your network as well.

I have not tested this my self and make no guarantees.

The LNB needs power to run the electronics and most splitters don't allow power to pass through.

@shannon02 Diplexers do.  DirecTV was very commonly diplexed with an antenna pre-swm