Sorry but if both forms are popups, there shouldn't be a problem
If the smaller popup is opened by clicking a button on the larger popup, it should always be on top.
If the second form was already open when the first form is opened, it will go to the back of the 'z-order'. However clicking the button on the first form will restore it to the top.
Very basic example attached.
Of course, clicking on the larger form will restore focus to that and hide the smaller form if located in the same place. But you can click the button again ...
OR just move the smaller form to one side.
OR change the larger form so it isn't a popup!