I bought a used replacement regulator on EBay. The gauge for tank pressure doesn't work, but because the one for keg pressure works I can still use it. I successfully carbonated and served from one keg a few weeks ago. However now I have two kegs hooked up to it with the gates open for both gas lines to carbonate two kegs. I have full tank I just exchanged yesterday. Last night the pressure to the kegs stayed at 20 psi for at least 6 hours. This morning it is at zero and I can't get it to go up again. Needle won't move. The tank and regulator are in the fridge with the kegs this didn't seem to be a problem when I used it with one keg.
Was there any keg lube used? I have had many issues with my kegs not holding pressure when keg lube wasn't used. It helps to lubricate the seals and make a stronger seal than it would using just the seals themselves.
It's cheap and the tube will last a long time. If you are unable to find keg specific lube, any silicone based food grade lubricant will work just fine.
Check for leaks, replace all the o-rings and maybe even the valves. Also get yourself some keg lube and make sure to hit the kegs with high enough pressure to seat the keg lids.