At that young age, you shouldn't have a problem mixing them at all, BUT... make sure they are vaccinated before you do. There's a reason for this.
Almost all established groups of ferals have never been vaccinated for anything, so when a virus strikes the group, they either die or develop an immunity and from then on, they
might continue to carry that virus and 'shed' it during certain circumtances - usually stress related. This only holds true for viruses the group has been exposed to, and there are many variations of cat flu, for example, that a specific group of ferals may never have com across. Both kittens may appear healthy now, but they could both be carrying antibodies to different strains and when mixed, they might shed the virus and make each other ill. By vaccinated them before you mix them, you will at least give them some chance of fighting off any nasties they may sheding.
I'm not saying vaccination will prevent them infecting each other, but it should lessen the effect of any illness they come down with. Nor am I saying they definately are shedding a virus, but with cats coming from different places whose background you don't know, it's a smart precaution to take before you mix them.
