I'm sure there are areas where only Comcast and TWC compete. If those are the only two cable competitors in an area, that eliminates cable competition.
There are a lot of ifs that would be needed to make that a problem. It would need to be an area with line of sight problems and two cable companies. Two cable companies is not common. Areas that could have two cable companies are good targets for fiber. I'm not going to say "impossible", but can you tell me how many of TWCs customers can get Comcast?
Sure, there are places where satellite is the only option. But there are people like me, for whom cable is the only option. Trees prevent satellite, not just for me, but pretty much everyone for blocks in any direction, and my telco doesn't offer TV. There is only one cable company in town. I'm no worse off today than people who can only get satellite would be if Dish and Directv combined.
Unless they are redwoods perhaps you just need more creative installers? For your narrow example a DISH-DirecTV merger would do no harm but do no good. The same would apply to the Comcast-TWC merger. You would still have one cable company.