Hi Ohmie, let us dust off this thread...
Do you live in South Africa? It does not say in your profile.
Let us start at the start.
The info on the web is (mostly) wrong due to a lack of first hand experience by many. M balfouri do not eat each other (cannibalize). I know it was said, and I quote "there is NO such thing as a communal Tarantula" (
Uncle Stanley Schultz). As I am the last to differ/argue with him, I do have the right to an opinion of my own, and in my experience, I have NEVER seen this happen. I do admit that I only started the communal rather recently, but all reports from others that did the same seem to agree. In the batch of ten slings I gave to the adult female to adopt, there is one or two runts, and even they are still in the group and are doing fine, even among their obviously stronger siblings.
Secondly, I would start the communal with the ones you have, but I do understand the financial risks. That again in my opinion, is the ONLY risk. I respect your apprehension, but I can assure you, they will not eat each other. I must presume they are from the same egg case? If so, just do it...
Then lastly, males and females of the species are exactly the same in coloration. There is no way you can even begin to guess at the sex based on outward appearance. Even adult males have the same colors, but can be distinguished by physical appearance. If you are going to keep them separately, sex them with an exuvium, else don't bother. If you have the koˈxones to put them in one enclosure, then you'll have to wait for the males to mature and start flirting with the girls.
For the record, I have witnessed two males seemingly aggressive towards each other in one enclosure, what I have NOT witnessed yet is what two mature females will do when they have contact with each other. I suspect a battle will ensue. This will be territorial and NOT to have a meal.