Is this a proper cage for two bettas?

i have a dual betta cage that has a clear divider in between them. The problem is that they obviously see eachother because it is clear and they are constanly flaring their gills and running into the divider. Is this cruel? I have been told that this may cause stress for the fish. Ive read some books and it says that they eventually ge used to eachother and accept it. Does anyone have experience with a dual cage with a clear divider?