In a tank that is 10 gallons or more (you might be able to find other dividers for smaller tanks, but I only recommend 5 gallons for each side), you can get tank dividers for about $8. I got mine from PetSmart and I use it to keep 2 males in one 10 gallon tank with 5 gallon portions.
There are some exceptions, but I don't recommend it until you've got your answers down for sure. I have a smaller (~3 months old when I bought him, 2 months with me, so 5 months old) who is simply not aggressive at all, and will not flare. He is in a divided tank and I have not caught him flaring once. (In fact, when I put the tank divider on wrong and the bigger, more aggressive fish got through it, he still wasn't flaring back when I came to fix it.) Even then, I wouldn't put him with another fish, betta or not (unless I was breeding).