It's 50/50 chance. If they become a pair then one will grow bigger than the other, but if they don't, they'll stay the same size and basically won't get along.
I tried to do this but both of them died because of not eating. Even until the end of their days, they still fight.
The chance is pretty good, considering you are getting them as juveniles. So instead of 50/50, it's probably more like 60/40 successful
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stupid_naso