Well, I was at a LFS
a few days ago. This lady came in with her daughter and wanted to buy a Betta. She had a one gallon Betta bowl, no filtration or heater.
Her daughter wanted another one, and this is what I heard from another customer.
"It should be fine, as long as each of them got enough space to call their own". "Oh a One Gallon? that is plenty of space for each to have their own space" . "If one is starting to go near the top and flash out it's gills that's sign of aggression and you will want to remove it than, but it shouldn't be a problem".
Ok to be fair that list bit of advice seems good. :-p But the girl (Teenager) that was selling the Betta said it was a good Idea to have two, and they should be fine together. When I chimed in, she told me "that guy has a degree and I think he would know more than you". So I guess me keeping a Betta for three years alive means I know nothing. :-/
Needless to say they got another Betta for their one gallon bowl. Is their a time where two Betta's can live together? Am I wrong? I know a female and a male can live together, but I know there is even limits for that..