Most pet store bettas are sold at around 6 months of age or older. Sometimes local places will have them even younger, but usually never less then 4 months.
It sounds like you've been lucky do far, but remember, they could easily go months without trouble and suddenly turn on one another. Obviously its your choice to separate them, but I can tell you for sure that if you do plan on keeping them long term together you will wake up one day to dead fish. Its inevitable that they will fight at some point, after all they were originally bred FOR fighting.
Ahh, yes, what you could do is separate one 10 gallon and put the other male in the other 10, then get 2 or more females(depending on how many other fish are in there, you could have a good number of females ;))and make a Sorority out of the 28. That sounds perfect :)