Hrm, looks like it may be divided down the middle.
Regardless, I personally wouldn't keep that as a permeant home for adult bettas, and would move them to something a bit larger if possible.
Until then, I would recommend carding them. Being in full sight of other bettas all the time is stressful, and they also get bored, eventually will not flare anymore. I use the covers of plastic display folders cut up to size, and slot them in.