Many vehicles have what are called Power On Self Diagnostics (POST - well, that's what they call it in the computer world anyway). When you turn the key to the run position (even before starting the vehicle) you will see movement out of all of your gauges in some manner (some bounce the needles, some sweep them all the way over to maximum readings). If the two gauges are moving when you turn the key on, I'd suggest it is in the wiring or quite possibly the sending units which are bad. The sending units are the parts of the system which tells the gauges what the read (some of them tell the PCM, and the PCM tells the gauges). If the gauges move at all when you turn the key to on, you know they are getting power, and therefor are not getting any signal to show a reading. If so, I suggest you tackle one problem at a time. The temp gauge should be much easier, as it is usually a single wire attached to the sender. It should look something like this:
Note that most of this will be either inside one of the engine water jackets or has a wire connected to it, so may not be easy to spot at first.
The gas level sender is another matter. It, of course, is located inside of the gas tank, which brings with it a whole other bit of trouble having to deal with what is actually in the tank. It would usually involve dropping the gas tank, unscrewing the gas pump unit, detaching the gas lines, and replacing the unit. Not a lot of fun.
IF, on the other hand, the gauges are not moving, you definitely have an issue inside of the cluster, or with a ground (as you suggested). You'll have to do further diagnostics to figure this out. You can also check the wires going back to the PCM to see if you have connectivity there (refer to a wiring diagram of your specific vehicle to figure out what is what --- Civic forums or a Google search can find this for you online). Since you are having the same issue with both clusters, it would be my approximation the problem does not lie in the cluster, but in some other part of the vehicle.