I have a viv with three females of three species. I have a viv with two females of the same species. I have one with two males of two species. I have one with a male and two females of one species (all around 8 months old). I have another with a single male.
It's a case of your personal choice, which should be based on the best husbandry for the snakes you keep. How you split them depends exactly what you settle on buying. My thoughts from what you've mentioned so far would be to have have them split by sex rather than species, I don't think your considering species that anyone would advise against cohabiting.