[Caveat: Newbie!]
Damn! That's my cunning plan blown!
(And IIRC, there is a French design being used which pretty much does this already.)
I am in the process of doing just what you described. Two home-made scrubber cartridges in one scrubber, electrickery to indicate to the diver when the second cartridge is taking over. Playing with size to get an acceptable WOB.
The benefits are just as you say - the chance of a breakthough/overbreathe is now miniscule. However, there is another benefit... (this is theory - haven't tested this yet...)
Dive your unit until you get the "beep" to say scrubber 2 has taken over.
Look at your watch!
At the end of the dive, swap scrubber 2 to 1 and put a new scrubber in at 2.
As a guestimate, subtract the time between getting the beep and getting out of the water on the last dive from 3 hours - this gives you an idea of what's left on that scrubber in case the electronics die.
The benefit here is that you will use every bit of scrubber 1 and will never be chucking unused slime away - without the need for a temp stick.
The other benefit is that you can dive beyond the beep - so now have 2 x 3hr scrubbers = 6 hours, if you want to push it. Stick a temp stick in there and you have 8+ hours I guess?
I'm just playing at the moment - there may be all sorts of pitfalls I haven't come across yet. Reliability of electronics, size of scrubber, life of slime in cartridges etc etc. But as was mentioned before, why not address the CO2 issue head on?