I wouldn’t split up the frames, unless you have signs of swarming urge - leave them where they are. It is very disruptive for the hive when you move frames around. After all, we wouldn’t like it if somebody came and rearranged all the rooms in our home! The risk is that if you move brood, they may abandon it if they are not ready to expand, then you lose a frame of perfectly good brood.
One thing to consider is putting the new empty brood box underneath the existing one. The advantage of this is that bees work hard to keep the brood at 35C. Heat rises. If you put a new brood box on top, all of that warm goes into an empty box, making more work for the bees to keep the existing brood warm. Also, when you consider wild bees, they pick a large empty cavity and build down from the top of it. If you add a second brood box underneath the existing one, you are mimicking what they prefer in nature.
Just my thoughts. I am adding my second brood boxes underneath the existing box this year.