Hi and congratulations on moving along with your bird. It must have felt fantastic when she came for that bit of apple!
I cannot advise you on where to put the cages of the two birds or how to combine them. Others are more experienced here. I can say that the danger of letting them climb on the other's cage is that the resident my come to bite the intruder. This means toes and feet can be damaged or lost altogether! I would advise against it unless there is no question of the safety - you would have to be carefully watching and right there to intervene. Recently I watched even my Budgie bite the foot of my Ringneck as he stuck it in her cage. When the Budgies land on the Ringneck's cage, he double-times it over to where they are. I will not allow them to remain long enough for him to reach them, due to the danger. This does not mean that the two birds (or a Ringneck and a Budgie, for that matter) cannot be happily and safely together, rather that there is inherent risk and it must be carefully managed, with the understanding that it may not be possible to combine them.
I would caution against caging a male and female together (and even more against caging two females together). In addition to any cage/territory/possessiveness issues, there are the twin issues of preventing breeding and promoting tameness. Your wild female is not likely to tame down to humans without a good bit of attention from you. However much attention you give, you cannot compete with another bird right there with her 24/7. The strong likelihood is that she will prefer the other bird. If you are unlucky, he could prefer her over you as well. Hand fed and hand raised does not really equal totally tamed.
If you are able to begin treating by hand, this would be the best thing you can do. Approach the bird however the bird accepts best, and offer a bit of treat. Frequently this will be done with the bird in the cage, you offering a treat from a little distance sideways down the cage. The bird gets to decide how close it comes to you. Initially you may have to "prime the pump" by wedging the food in the bars after an unsuccessful attempt and leaving. Don't overdo that, or the bird will have little incentive to come to you. However, you probably will need to introduce the concept of going there for that thing, so a little to start is fine, even needed. If flying off with apple was exciting for you, just wait till that bird takes some apple from your hand. You'll be over the moon.
