i wanted to keep parts separate so i could have different levels of subdivision, and im using proxy smooth so i dont think i can move the edge verts, ideally you could select edges you didn't want smoothed, or crease them so they actually stay fixed even with a smooth proxy you can do that. you have to keep the "cage" low res polygon at the connecting parts the same amount of border edges. you can use the snap to verts to line em up between part a and part b. now as it gets smoothed the border edges of part a and part b behave exactly the same (if they have the same amount of subdivision setting in the smooth proxy). border edges don't move, they stay in place just get round. having it that way would be equivalent with a subd surface that has 2 different parts of faces that touch.