Hi everyone, I will do my best to help here as much as I can, and yes, that includes the plants!
Let me start by saying please don't put females together. I have seen what they do to each other, and where anyone gets the idea that they are good together I just don't know. When I worked at the store my boss and I found conflict on this topic, and it was when I proved him wrong that he finally gave in. Breeders keep females together longer because the fight instinct isn't as fierce at a young age, but as they mature, the females can be just as nasty if not more so than the males. They chew each other's fins to bits, and it doesn't take much, especially in a small tank such as a 10 gallon. We couldn't keep 6 females together and healthy in a 40 breeder, though the boss insisted time and again we try it. I have no reason to believe it would be any different or better in a 10 gallon. Once the fins get chewed they are prone to bacterial and fungal infections which will usually kill all but the dominant fish. I got tired of seeing it happen so I had a good fight with the boss, pointed out the money he was losing on dead fish, and won my battle. For as much as the cups seem inhumane, they really are a lot better than being chewed to death. Because bettas don't rely on oxygen from the water to breathe, they are able to withstand some pretty stagnant conditions before they even become ill. No, this doesn't make it right, but if they have clean water and good food, and some kind of interaction regularly, they are pretty content. Space is not something they require a lot of. The long fins of a male fish make it dangerous to keep it in a very large or deep tank/environment. Those fins are not made for swimming, they are made for looking big and dominant to chase off other fish. The length makes them very weak swimmers by fish standards, and if they can't easily swim to the surface for air when they don't feel well for some reason, they actually can drown.
One other thing I want to touch on before I address your immediate situation. Bettas are not
community fish. There are very few situations where this can work, and is dependent on the personalities and habits of each fish as individuals. There are some bottom dwelling critters like ghost shrimp and dwarf frogs that work very well with most bettas, but other free swimming fish usually end up causing the early death of either the betta or the other fish. Other fish tend to also have other needs in a tank than what is normally good for the betta, such as circulation. With heavy circulation you create oxygen in the water, which the betta doesn't need and would have issues with. Take away that circulation and you create a dangerous situation for the other fish. So, you can see why there are good reasons to keep bettas alone and in semi small containers with little or no water current from filtration and such.
Ok, with that said, there is no reason why you can't house 2 males in a 10 gallon tank, provided it's set up properly. One thing that will be mandatory is a tight cover that prevents the fish from jumping into each other's sides of the tank to fight. They will be visible to each other, so the temptation will be there. Natural instinct for them is to jump from puddle to puddle or place to place to reach an opponent to defend territory. Expect this, and if together, expect one to die if not pulled apart fast enough. It only takes a few minutes for them to damage each other enough to cause death... whether by direct injury or secondary infection to a wound. Lots and lots of plants. They don't have to be live, but live will help keep water quality a little better. Live plants don't have to scare you. If I know what kind of light you will have over it I can make a few suggestions that would be easiest in your situation. Hornwort
is usually tolerant of most lighting conditions in a small tank, and can handle the temp just fine. It grows fast, so you will need to trim it out regularly, but it's good for the fish to play and hide in, and it has no root structure that needs to be planted.
Make sure to heavily decorate the divider on both sides. This will prevent the most stress from seeing each other too often, and will create a barrier when one fish needs to get out of sight of the other. If you can also provide some small tunnel or cave type structure for each to call home, that helps a lot too.
Outside of that, keep them warm (78 - 82 is best temp range), keep them clean, (vac both sides to remove waste regularly) and keep them well fed (live brine shrimp, live black worms, frozen brine that is completely thawed for at least 15 - 20 minutes, betta pellet food, freeze dried brine or plankton, if they will eat it, offer it to them. I had one betta who ate frozen formula 1 food, and he was always healthy and with good color. Snails make good tank mates, although some bettas will eat small ones. This is a good snack for them if they do, healthy, and good cleaning for the tank if they don't. Snails win both ways. If you work with a mystery snail or 2 instead of ram's horns, you won't have the population issues to deal with. They are different in how they lay their eggs, and its easy to prevent them from hatching. I keep ghost shrimp with all of my betta tanks, they make good maintenance critters, are fun to watch, and don't bother betta or snail. Shrimp need seperate rock structures to hide under, but its worth the extra few chunks of rock for them to thrive.
I hope this about covers any questions you had... if you have more feel free to ask away.