It just depends on the bettas if they will calm down, just as it depends on their personalities if they will be ok in a community tank. I had two male bettas in a divided 5 gallon tank
oops: my first two, and a 5
gal wasn't big enough for both of them, really.) But surprisingly, they got along very well.
When they died, I got another 5 gallon tank, and put the first 5 gallon tank and the new 5 gallon tank beside each other, so each new betta could have his own tank. The bettas that I had in these tanks flared at each other a lot, at the corner where they could see each other. My second two bettas weren't calm like the first two I had.
So to answer your question, it just depends on their personalities. Try to find a more opaque divider (craft plastic forms with square holes for yard or thread was what I used, in place of the clear divider that came with the frame). I also considered using the aquarium background sheets. You could try that, but be aware that it may cut off the water/heat/filter flow to the other half of the tank. You may have to adjust it and work with it a bit to overcome those concerns. I think the best thing to try would be the opaque divider, and plants along the divider on both sides. Also, some plants within each side of the tank may be helpful too, so the bettas can "get away" to their own area and hide if they want.