I guess we need to clarify here. If you are talking about a receiver DECA, like what is currently hooked up to your H21, no you can't hook that up to the same coax as a C31. Receiver DECAs have a bandstop filter built in that would block the DECA signal from getting to the C31. Not to mention the C31 can't power the receiver DECA, so it wouldn't work either unless you hooked a power supply up to it somehow.
If you are talking about a wireless CCK, then yes you can do it. The Wireless CCK has a splitter built into it, one side of the splitter is used for the DECA circuitry, and the other side of the splitter goes out to the coax port so you can run it to your DirecTV receiver. The Wireless CCK also has it's own power supply so it doesn't need a receiver to supply it with power to function.
If you are doing what I think you are doing then you need to get a Wireless CCK. To hook everything up you would take the coax cable that currently plugs into your H21's DECA adatper and hook it up directly to the Wireless CCK's coax input. You would then hook another coax cable up from the wireless CCK's coax output and hook that up to the C31. Then you would run an ethernet cable from the Wireless CCK to your ethernet switch, or directly to the laptop dock if that is the only item you want on the network.