A Revo uses two 1.35kg/3lb scrubbers. These are monitored using their temperature monitoring (RMS) which can properly assess when the reaction 'front' reaches the second scrubber. It's surprising how long you can dive on a single scrubber; I've never yet managed to dive long enough to start consuming the second one. As a result you throw the first used scrubber and swap the old second scrubber to be the new 'first' scrubber and refill the consumed scrubber to become the new second scrubber.
I've (in my limited time on the Revo) only ever replaced one 1.35/3lb scrubber at a time for every dive -- max dive time circa 3h. With that low cost there's no point in pushing it.
One of the principles of having two scrubbers in series is reliability. Should the first scrubber break through or be consumed, the second one's there to save your dive/life. The monitoring will pick this up and alert you. The key to this is mixing the gas between the scrubbers; this means the CO2 from a breakthough would be mixed across the whole face of the second scrubber and not concentrated on a small area.
One wonders with good monitoring if those scrubbers could be reduced in size. However, reducing size would probably mean a higher WOB (if narrower) or a shorter scrubber life (if shallower). Maybe that's why there's no half sized rebreathers.
