Create a template override for /modules/mod_eb_events/tmpl/improved.php or default.php, depending on which layout you're using... (alternately, you can create an override by simply coping the file to /templates/YOUR_TEMPLATE/html/mod_eb_events/ ).
Within the file, look for the line that reads:
Code:
echo JHtml::_('date', $row->event_date, $config->event_date_format, null);
and add the following on the next line:
Code:
echo ' – '.JHtml::_('date', $row->event_end_date, $config->event_date_format, null);
This will work to produce the output you requested, but would additional code would be required before it would be ready for inclusion in the core release (a toggle switch in Config to choose to enable/disable; perhaps special handling for not showing if start and end date are the same; perhaps special handling for same day, but different start and end time *if* times are set and *if* the date format string includes time...)
