Female bettas need to be kept in bigger groups due to their aggression. having only two fish you will have problems. i know from experience. i had two in a large community tank, they were fine for months, then something changed and the aggression kicked in. i think the minimal size for female community is 5 or 6 females in a 10 gallon (40L). 2.5 gallon is fine for one betta, but i would not risk two.