Do you plan on keeping the gas on both kegs for the remaining 3 days prior to serving?
Yes, I am.
Can't you take off the regulator and weigh the tank, subtract the tare weight and see how much is left?
Now that's quite reasonable. Short on the time needed to disassemble my system, however.
They're expensive for what you get, but the adapter + 16 gram CO2 cartridges are good for emergencies and for serving when you travel.
I do have a portable set up for 2.5 gal tanks with a paintball tank, but only one regulator. I suppose if needed to, I could switch between kegs to keep enough pressure for serving.
But I think I'm just going to buy a brand new tank tomorrow for a backup so I can avoid this in the future.