that sounds like a great idea.... could one heater heat all the compartments and the filter cycle the water good enough?
I would do a few things...
If you use a canister filter, with the spray bar running the length of the tank, you could direct one of the extension poles to spray right, and one to spray left. That would give even water flow.
A second option would be two AC 50's, both on the short side of the tank, one filtering, each long side of the tank. It's tough to say how well it would work, but for a single beta in each compartment, I would have to think that with a little water movement, and a live plant in each compartment, you would probably be fine.
Secondly, if you drilled enough holes, it would allow for water flow to get through the tank, and third, I would use two heaters, one on each side of the tank, both rated for 30 gallons, just to make sure.
Also remember, the downside... to do a water change, you now have 8 little tanks to vacuum out!