They have their moments.. they don't need to flare constantly. I have most of mine within sight of each other and other then when one seems as if s/he is eating will the other get excited. Otherwise, for the most part they ignore each other.
Bettas are actually pretty social, so seeing one another constantly isn't a bad thing, and some actually do better when in view of another. As long as the tank(s) have plenty of plants and a cave of sorts- being next to one another daily will usually give them exercise and sometimes they will be more active overall.
my female is always flaring at her reflect. Is it a stressful situation for her? One of the tank wall is black so it makes the other walls look like mirror. I dont know how to manage this situation.
@Meve:No. This isn't a stressfull situation. As stated above, bettas will flare until they get "used to" each other. You have to make sure that they arent "obssessed" with flaring that they won't ever go away from the source or won't eat because of how busy hey are flaring. They re healthy. If they are flaring and losing color or seems to be at the bottom of the tank with clamped fins, the re stressed. Usually if they are flaring back, they are not stressed.
@fabian: possibly stressed, worn out, or fed up. lol. You need to accustom them to the length of flaring. Say you just got a couple of guys that were side by side in cups. You can't just flare them. You need to give them a day or so to regain their self esteem. Then you flare them for a minute or less. The next day you can flare them longer. And so on. This specially applies to the younger males - the recently jarred.
Stress is usually shown by less movement and or color fade/stress stripes. But if they're still active and vibrant, they've probably figured out that it's useless. If they are stressed, this could lead to being afraid of their opponent's color. Try isolating them for a few days. Then in the next couple of days flare them to a mirror for a minute or so (you can do this twice a day). After that you can flare them to each other.
If you want them on "alert" mode but don't want them to constantly flare, don't totally open the whole divider. Just lift it up an inch. You can also use a divider with a pencil sized hole in the center. This will also help the less aggressive because he could hide..... DO NOT open the top part of the divider because bettas fight on the surface so they will constantly see each other. I flare my bettas with this method for about 15 minute daily. Sometimes I'd let them flare until I'm done with all fish care work (1-2 hours). But you need to train them/get them used to flaring beforehand because to them, it's fighting.
If you have several colors, alter their flaring partner from time to time because they will often get fed up or stressed.