Tandem Trlrs are safer when/if you have a tire blow out,...it won't go squirrely on you,...plus they distribute the load over 4 tires instead of 2.
Disadvantage??--they're harder to turn and maneuver by hand in your driveway or garage, and your doubling the expense when the tires need to be replaced.
I've owned both and I prefer the Tandem Trlrs for 20ft boats and longer, although I've had them for 19ftr's too.
Mac