Now I will go off my tangent :):
Divided tanks are cool because they can hold more than one betta.
However they present some risks: Your two bettas are sharing the
same water and living condition. If one gets sick, he will infect
the other. In a way, two bettas in a tank are like two convicts
chained together. You'd better make darn sure both your fish are
very healthy at all time. If one of them shows ANY sign of sickness,
it should immediately be removed and BOTH bettas should be treated.
At least I would add a good disease preventative to the healthy
looking betta, such as Bettamax, just to be on the safe side.
This problems is especially troublesome when you have a large betta
barrack, or drip system, or anything that houses a large number of
bettas all sharing the same water. These are bound to
"crash" on and off, where a disease will take hold of a
few bettas and them quickly spread though the entire colony and
decimate your stock. Owners of such systems must be very attentive
and stay on top of their apartment complex tenants LOL, big time.
True, UV sterilizers help some but they do not eradicate all
bacteria. Regular water changes, proper maintenance, quarantine of
incoming fish etc are a must for any such large systems.
Now back to you. I would remove
the 'not so healthy' betta and keep him in a large jar, add some
bettamax to his water for a while. Find a more suitable and active
companion for your other betta. Remember to quarantine any new betta
first before adding him to your dividing tank, so it doesn't bring mayhem
with him! If you want your bettas to remain active longer, you
might consider purchasing them from a breeder instead of a pet
store. You can get them young that way (3 months old instead of 1+
year old). They will last you longer. Goodluck!