Do you want to do this in one specific place or for your entire application? Making this work globally could be tricky.
If you just need it in one specific place, you could look at LocateControl in stdlib and see how it handles processing the Enter key. It uses SetWindowProc and then handles the window messages in the Enterproc method. You could do something similar. There is a SetFocus function that you could also use to place the focus in the other control as long as you have a window handle.