We do need a little more information than that you checked the water.
What gender are they and where are they? Whether male or female, they should be in separate tanks because males go with NO other Betta(including female) and females go in groups of 3 or more only. Are their tanks at least 5 gallons each? What exactly did you check the water for?
What is your idea of 'stressed'? A stressed Betta will demonstrate dark horizontal stripes. It is more common in females than males, and you may not be able to see it if your Betta is light colored or, obviously, cellophane. Vertical stripes in males can resemble stress as well. In females, it is fine and represents "horniness" or wanting to dominate. Clamped fins are also a sign of stress.
If your Betta is not active and eating less he/she could be sick. Even if the water conditions are right. You'd probably have looked for Ich already, but shine a flashlight at your Betta and look for Velvet. Your two Betta could just be bored with their environment, also, or maybe they dislike it. If they are on a stand that has many vibrations like a dining room table or computer desk that could be the cause. If your Betta are next to each other, put something in between the tanks so they can not see each other. If they are not next to each other, put them next to each other.