As built in functionality no this cannot be done without some work. This is already a feature in the works which will be coupled with analytics for tracking opens etc.
You could create a solution for your needs like so though until the functionality is bnilt in.
Create to modals (eModal-1, eModal-2)
Use a bit of javascript to generate a random selection after 6 seconds and open the modal as well as fire a google analytics event to track which was opened, which could then be used as part of a goal funnel to track which works better.
the JS would look something like.
setTimeout(function(){
var modal_id = Math.floor(Math.random() * 2) + 1;
// Fire google event code here tracking which modal will load from the modal_id variable.
$('#eModal-'+id).emodal('open');
}, 6000);
Hope this helps, please take a moment to rate and review the plugin and or support here