You can do anything you want.
The only NEC regulation is that the grounding block is bonded to the electrical ground pointwithin 20' and normally Telco, electric and cable services all enter in the same area
Not sure why you think it is dangerous to put both boxes side by side, but to each his own. If you pull your own cable, have TWC come out to install their special F connectors on both ends and check signal levels. Then you can't be blamed if your connection acts up down the road.
I don't have a wireless card for my PC so I need it to be connected to the ethernet cord . That cord is running on the floor in my hallway along the wall and in to my nursery. I just don't want my son to mess with the box or cable , as it's in his bedroom. It's up high and the cords are secure but I still think of it stupid to be in there. Even I I unplug my box and put it in the same room as my PC, there with be the cable to the box running down my hallway instead of the ethernet cord. Just trying to make things easier and safer. I watched the technician literally drill a hole in the wall and put the cable thru ( an old phone cord went through that spot prior ) after that he put the plate over it on the inside and connected the box. When he went outside he connected that cable to the pole that's outside and only three feet from my own bedroom where I want the cable to enter through. It was guick and easy . Something that someone that can use a drill can do. I was curious and called TWC and they will charge me $90 plus there fees to do the job. That's a ridiculous price for something so simple.
No need for a wireless network card these days. You can spend about half that much at your favorite PC and electronics store to purchase a WiFi adaptor that plugs into a USB port on your computer. Linksys has even been giving them away free with some of their newer routers.
All things considered, $90 is a fair price for rerouting your cable connection. They gave away the first install because you were a new subscriber, but they can't afford to do that work for free every time somebody wants to move a cable modem. If they do, the rest of us are paying for it.