Echo 2 values from one key in Array with get_post_custom

The main problem with your code is that you only store the value and not the venue name a simple fix would be to change your vlaue to hold the venue name as well ,
something like:

array(
    'name' => 'Venue',
    'desc' => 'Venue of Event',
    'id' => $prefix . 'event_venue',
    'type' => 'radio',
    'options' => array(   
        array('name' => 'Pine Gallery', 'value' => 'Pine Gallery***http://www.pinegallery.com' ),
        array('name' => 'Spruce Gallery', 'value' => 'Spruce  Gallery***http://www.sprucegallery.com'),
        array('name' => 'Oak Gallery', 'value' => 'Oak Gallery***http://www.oakgallery.com')
    )
)

and the other problem with your code is that you are looking for $event_custom_meta['event_venue'] and there is no field named event_venue since you define it using a $prifix so it should be $event_custom_meta[$prifix.'event_venue']

so now $event_custom_meta[$prifix.'event_venue'][0] should have but name and URL separated by ***
so split it to an array and you will have bit values, something like this:

$event_venue = $pieces = explode("***", $event_custom_meta[$prifix.'event_venue'][0]);
echo 'name: '.$event_venue[0];
echo '<br/>url: '.$event_venue[1];

Hope this helps