Someone just asked me why it isn't possible to calibrate all four sensors.

The reason is simple, its not a bug, its a feature!

What happens is that the other sensors are calibrated to respond the same as that one (0 in software) 1 in the menu. This means that if that one has a different response curve, after calibrating all four sensors will respond the same way. That actually means its possible that the sensors are responding with less accurate results and that's actually a good thing!

When calibrating a set of carburetors or throttle bodies, we need them all to respond the same way over the full pressure range the engine can produce, so the sensors need to do the same! We are only interested in differences between the cylinders of the same bike, you are not getting a reading and transferring that to another bike somehow. No we are only interested in relative differences between the sensor readings. All sensors could be off by 50% and it would still work as long as they were all off by the same 50%!
Luckily these sensors seldom have more than 2% variation and that is easy to calibrate out. Actually doing this is what makes CarbOnBal crazy accurate. 

Don't believe the hype on those expensive electronic units that claim to not require calibration. They never bother to explain why... And no "Advanced electronics" is not an explanation. The only system that actually doesn't require calibration is a mercury manometer setup even the old aquarium hoses (at least in theory!)... The others are just always off by a bit, but they should be within usable tolerance anyway, certainly well within the manufacturers specs. 

Compare it to using the old aquarium air lines to tune. You don't care how much liquid is in the tubes as long as all the tubes are the same diameter and one line is not pinched, for example. Nobody takes their aquarium hoses in to get them calibrated, there's no need.