well i would just like to say getting 2 bettas divided wont be such a good idea since they hate each other, and your betta which you have right now will be alittle stressed when he notices his tank is half the size it was and has less swimming room. 2.5 is an ok size like imagine you living in a walk in closet your whole life instead of a nice big room
i think you should keep it how it is.I know your tank which is high length wise not width so the bettas could swim a little side to side but mostly only up. Do what you want to do though its just my opinion
