i believe you can replace the valves but its probably easier and cheaper to replace the tank. to test the valve(or at least the guages, hopefully both are working) you could measure the resistance at both guages, one will have wires on, one wont but this is where you need to test.
make sure the tanks are set with the valve part about 30 deg from top centre. if one is not emptying then as a temp measure you could turn it off, this will make it safe. it could just be the solenoid on the tank, if there is one for each tank, compare resistance on them. once you have found the faulty tank and isolated it, you can remove it and put in another tank, use a compression fitting prolly 6mm to join the pipe and a new piece of pipe to make up the section. or the best method would be to empty the good tank or turn it off. undo the t's and re pipe from there. depends on ease and accesibility but the less joints = less leaks possible.