Resizing custom control

[aquafatz]

I'm working on a mfc application, where there is a main dialog with several custom controls on it.
I have two custom controls, one below the other.
I want to add a horizontal splitter bar between them, such that when moved down, the top control increases in size(i.e. in length) and the bottom control decreses in size(i.e. length) and vice versa.

I'm confused about the logic I should apply. When the splitter bar is moved, how do I change the top and bottom controls accordingly? Should I re-draw them with the new dimensions?(But isn't redrawing for every move of the splitter inefficient and may lead to flickering?) Or is there a way to just increase/decrease the length.
I also came across DefWindowProc. Can it be used in this case?

Thanks a lot for the help!!