If you have two boxes targeting the same page – the topmost lightbox gets preference. In the image below, Box 1 would get preference over Box 2.
If you want a lower priority lightbox to have a higher priority, duplicate the lightbox so that it is lower on the list.
There are other things you can try, too. For example, if you set a rule like Display until Form Submit on the topmost lightbox, then the lower priority lightbox is displayed instead.