You really shouldn't be dioplexing Cable Modems and Dish on the same line at all. The less signal to the modem, the more potential for packet loss which means slower internet speeds.
Think about it, when I had comcast installed for my internet, the first thing they did was replace the drop to the house and run all new wire to the cable modem. In addition to that, everytime I have had them out for a service call the first thing they do is replace the drop to the house.
You really need to run a seperate line....
Now with that said, I only dioplexed a cable modem for a customer once in the 8 years I have sold DISH Network. That was only after I asked this one lady if she was planning on disconnecting her cable 3 times before I used the existing wiring she already had run in the walls. The responce I got was that she was getting rid of comcast completely.
Then as Im pulling out of my driveway she runs out and tells me her internet is not working. Then when I ask her about cancelling comcast, she changes her story on me and says she is keeping them for just internet
So I went in there and installed 2 dioplexers and had the internet up and running 5 minutes later.
But reguardless, I was up in her attic fishing cables all afternoon, if I would have known she was keeping Comcast internet I would have fished a 2nd wire down her wall and installed a double plate.
But I would really like to know why installers have to take short cuts by installing dioplexers with a cable modem anyways. I always tell my installers that if they came to the house and it wasn't pre-wired they would have to install all new cable anyways. So what difference does it make if Comcast is using one of the pre-wired jacks?
Usually when your dealing with a 2nd floor, the issue is getting the wire to the attic, and not down inside the interior wall. So just take a feed from another room not being used and splice the cable in the attic, or run a 2nd wire up the side of the house.
There are ways to make this work, people just don't want to do the work.
The best calls are from customers who get Dish and the Dish installer uses the existing cable installed by the cable company. 3 years later, the customer orders a cable internet from the cable company and the cable installer refuses to run new cable saying the satellite installer should never have used their cable and installed his own.
The stupid customer lets the cable installer get away with disconnecting the Dish to hook up the cable modem, and the customer calls DISH Network to complain and to get them to run a new wire for FREE.
What would the cable installer have done if the cable was not already there when he came to the Job?