John, Have you ever had a betta get through or over the tank divider?
Just curious as that is one reason I have never tried a divided ten
gallon for two bettas.

At first, yes - particularly when I would have a female on one side and a
male on the other, the female would often find a way to get over to the male
(they are fantastic jumpers and are often slender enough to exploit any gap
between the divider and the glass). I've posted here before about such
male-female pairs in divided tens and noted that after six weeks or so in a
divided tank they can then live together quite happily (just pull the
divider) but usually don't attempt to make baby bettas. I had two males get
together once, but they didn't actually fight so I was able to separate them
without anyone getting seriously hurt (they were divided in the morning,
together when I got home from work that afternoon - I never did figure out
how the intruder managed to get around / over / under the divider). This
makes me think that a pair of males that were well-established flare buddies
might be OK together in a large community tank but I've never tried more
than one male betta at a time in such a tank.

I make my dividers from plastic report cover binders and plastic mesh (both
found at Wal-Mart or hobby stores). Make sure the divider has good, sturdy
support on the sides (I usually attach the binders to the sides with a bead
of aquarium-safe silicon sealer) and that it is well imbedded in the
substrate, and make sure that the top of the divider is well above the top
of the water (you may have to go with a lower water level than usual) and
you shouldn't have any trouble keeping your bettas apart.

--
John Goulden
mostly goldies, guppies, swordtails and bettas