help with splitting and combining

so i am relatively new to this stuff but have seemed to do ok so far till now. i watched all the videos but none really showed me what i needed to know. i am trying to model a frame, in which the two main rails have mirrored dimensions. i made the first rail, mirrored it, and then used the split feature. basically what i see so far is it put a dividing plane between the two....essentially at the mirror point, but now i can't seem to separate the two to make parallel. did i miss something or is that really not possible and i actually will end up needing to make a whole new frame rail with the opposite specs then combine? to dig a little deeper, i was having trouble mating objects as well. i made a roller, then a roller bearing, but had a heck of a time inserting them into the roller part. one went in easy, but when i rotated and tried to insert the other, it wouldn't do it without removing the first one. what am i missing? any help is greatly appreciated! thanks in advance....