If you have 2 campaigns targeting the same page, the topmost lightbox in the Campaign List takes precedence.
Other factors may affect placement as well. For example, if you set a rule like “Display until Form Submit” on the topmost lightbox, after it submits then the lower-priority lightbox will always be displayed.
Typically, when two campaigns are eligible to display, they will do so back-to-back. You can prevent this on the same pageview with a Master Rule. An exception is Inline Campaigns, which can display at the same time as Pop-ups, Sidebars, and Banners. In addition, Inline Campaigns configured to “Always Display” in the Widget Accordian will always display, regardless of Campaign conditions or priority placement.