Using a splitpane in a dialog

In your favourite dialog editor, if available, select the user-defined control or use a container or frame to reserve space for the splitpane control.
Once you have moved the place holder control on the dialog, place over that the two controls to be displayed on the sides of the splitpane, without caring too much about position and size, as the splitpane would take care of that, add any other controls needed by the dialog and then save it as a resource script.
Then open the .dlg or .rc file you created with a text editor and find the statement of the user-control or any other control you used as splitpane place holder. It will probably be something like:
CONTROL "some text", <id>, <x>, <y>, <cx>, <cy>, <class>, <style>
Now modify the statement:
  1. Replace the control text, if present, with an empty string (""),
  2. do not modify the id, position coordinates and size of the control (unless there is any problem with them),
  3. replace <class> with WC_SPLITPANE,
  4. ensure that the WS_VISIBLE and WS_TABSTOP styles have been specified and add to them any other specific splitpane style you may find suitable for your purposes.
Regarding the splitpane children, you can safely ignore their size and position. Just ensure that their IDs are unique within their parent dialog.

Finally, to make the splitpane work properly, in your dialog procedure, on dialog initialization, you have to notify it the IDs of its child windows, by sending the proper PM message or by using the macro defined for that purpose in axSplitpane.h:

rc = dSplitSetChildren(hwndDlg, idSplitPane, idLeftTopPane, idRightBottomPane);
In case you need to create nested splitpanes, just overlay the control you are using as place holder as you would do with any other kind of control and then modify the resource script statements as described. Finally, when calling the dSplitSetChildren() macro, use the id of the nested splitpane.