I have two gas hot water heaters running in tandem. Believe it or not we run out of hot water. One of the heater has the pilot on but will not turn on, even if I turn up the temp. compared to the other one the tube that runs from the temp dial, the part that sits by the pilot light looks like an ember... is this the problem?

If your talking about the thermocouple, yes it is set up so the pilot light flame is on it to proof that the flame is hot enough to keep the pilot light lit so your water heater will work. You can try cleaning it with either steel wool or a very fine emery cloth sandpaper do it carefully though not to break it, just to get the soot off it to clean it up so it works. Make sure when your done that it is in the plot light flame so it can proof the flame. Tell me if this works for you . Thanks and Merry Christmas!

Jack is correct about cleaning the thermocouple but it sounds like it is working. Your real issue is the other heater so devote your attention there. How big are these heaters and how many persons in the home? Lets find out if two heaters are really needed. The bad heater has a pilot but may have a bad thermocouple but please answer my questions before we try to fix it. Thanks

If the pilot light is staying on, no matter how bad things look, cleaning the thermocouple won't fix the problem. I would look at the larger tube coming out of the temperature dial. It seems unlikely that the gas tube would be plugged up and stopping the gas. If it is clear, the trouble is in the main control. Shut the gas and water off, disconnect everything from the box with the temperature dial. Drain the tank. Unscrew the box from the tank. It should have a rod sticking into the tank. Unless it is heavily covered with lime or other crud, take the whole thing to a hardware or home center and see if they can match it. You may even need to hit an appliance parts store.