If you're going to plant this with real plants then for the love of the gods change the gravel to a plain variety ( as in no artificial colours ) , otherwise it will render "natural" planting rather pointless. Sand is a really nice option as well , plants do very well in it. I'm not a fan of most ornaments but it is your tank and your choice of course, since you have two then each betta could get an ornament to play in/rest on among the plants.
Good plants to go with if you're a beginner are Elodia ( which you already appear to have on the right ) Cabomba , Aponogentons, Cryptocorynes, Hygrophillia Polysperma, Moss balls, Java moss and Java fern.
I am successfully growing most of these or have done in the past, and I am no gardener! Very easy most of them.
The moss balls are the absolute easiest as you just chuck them in and leave them to roll about doing their own thing. If nothing else I can suggest you start off with Cabomba, more Elodia ( these two will merely need the tops trimming when they reach the surface ) and mossballs.
You can buy tank dividers on Ebay. Make ABSOLUTELY certain that the divider is tall enough and well above the waterline, so that it is about 1/2 a centimetre away from the cover glass/ splash tray or hood. Bettas have been well known to jump over dividers that are not tall enough, and fights ( often esulting in deathfrom stress if nothing else ) were the inevitable result .
I would suggest a spray bar if you are having one filter to run the whole tank. Face the holes of the spray bar at the back wall of the tank, and pointing down slightly. This will deflect the current and keep the surface movement to a minimum, which should suit most bettas.
If you are not making a separate compartment to house the filter and will have it in with one of the bettas , then ziptie some very fine mesh around the intake holes to prevent long fins being sucked in.
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