If the second panel is a true "child" of the first one, there is no problem in keeping it always visible: it will remain visible inside the parent panel as long as you don't hide it.
To load a panel as a child of another, you must pass the handle of the parent panel as the first parameter to LoadPanel, this way:
childPanelHandle = LoadPanel (parentPanelHandle, "myFile.uir", theChildPanel);
DisplayPanel (childPanelHandle);
The panel will be displayed "inside" the parent panel (i.e. it will be possible to move it but it will remain confined to the area of the parent panel).
On the contrary, if your second panel is really a top-level panel I'm afraid there is no way to put on top of the first panel if this one is declared to be "always on top".