Female betta aggressive toward new female roommate

Hi I got an unexpected female betta fish as a rescue and tried adding it to my 10 gallon that has only one other female betta fish in it, she started getting extremely aggressive towards the new visitor so I had to put her in my hospital tank that I usually put injured fish in(5 gallon) temporarily, I’m currently saving up for a 20 gallon and I’m going to put a divider that they can’t see through so they have there own space, if anyone has a tank that they can sell on Vancouver Island that would be great :) other than that any suggestions to keep her happy? I don’t want her to be bored at all, any suggestions would be great :)