At least I'm not alone
Sorry if this comes across as harsh but ... There. Is. No. Way. To. Stop. Fin. Biting. Period. Betta owners either learn to live with it or rehome the biters. It breaks my heart to see so many people stress because think they have it figured out only to have the biting resume.
It's not your fault. It's a behavior you can't control and, after reading about it for so many years, it seems a high percentage of Betta in open and/or shared barracks have the problem. As Hallyx says, a tank isn't properly planted if one "can glance over and easily see a Betta." Then add that Betta in a divided tank share more than water; they share stress, too. And phermones.
As Betta are not bright light/lightly planted fish, you could try muting the light and adding a jungle of plants but you'd still have the barrack's problem. Perhaps experiment with one of the biters in a single, muted-light, jungle-like tank.
Knock wood, since I've started keeping tanks like these fin biting has stopped. When I had a divided tank it was this planted in both sections. Top and bottom photos are same tank but bottom photo is when the 20 long was home to Dexter, an OHM. Notice how many darkened areas there are?
Thanks Russell - I'm not so much stressed about it, as just frustrated *lol* it seemed they were doing so well for the last 2 weeks... but then tonight it just changed... *sigh* In any case... I have been keeping a record of any changes I've made to their tanks [you're welcome to move this thread to journals - if it's better suited there]. I do see your point though about increasing the plants and providing more darkened areas for hiding.
And while I know there isn't much I can do to stop the biting, I suppose this is a good place to document changes etc. so other members might also find solace in the fact that many of us have betta's/fish that nip their own fins.
I will post a few things that have worked/not worked for my fish (at least for 13 days any ways :lol

- perhaps it may help others (even though there isn't much that can be done if they really want to nip/bite their fins - I've accepted it for the most part, but it's more of an "ok, so you did it again *sigh*" kind of frustration)
Their water is heated to 30C (86F) - it is cleaned weekly; with a 30-45% water change; stress coat is 5mL and no AQ salts (even though the box says it can be added to water that has been changed), rooibos tea @ 1hr (for darker water)
For both boys:
- I have ruled out solitude - it was worse when they weren't around other fish. So, I have moved both of them around between the other betta's I own so they have a variety of fish to look at and be kept occupied.
- ping pong balls - they don't play with them, but on occasion Killian likes to blow bubble nests around it
- They didn't seem to like heavily planted tanks w/ no ornaments - I tried, but they both nipped more (especially Tony)
- lights are on for about an hour (max)... usually I just leave their light off - the only other light source around them at night (mostly) is maybe from one of the other tanks or the room light) - lights bother Killian more than Tony
That being said, I'll most likely try and change them around again and see how they go... I guess I have yet to find their "tick".... guess it was a little too hopeful that they would stop after 13 days of not biting *lol*