2.5 gallon is perfect for a single male betta, nothing else. Only 5+ gallons can be divided. 2.5 gallons can be heated safely with a small watt adjustable, submersible heater, and can even be cycled if you really want to....though I personally just do the 2-3 times per week 100% changes instead of bothering with cycling in my tanks smaller then 5 gallons.
Females need to be in groups of 4+(the more, the better) in a 10+ gallon(the bigger, the better) densely planted tank with tons of hiding spots. If you keep two together, one will kill the other. They can be just as aggressive as any male, and can only be kept in groups so the aggression is spread out. However, even that is extremely risky and doesn't always work out.