Actually the pivot point would be the castors on the wheels you linked to. The issues you might have are:
, be sure you are within your state's limits.
, only some states allow double towing, not sure about the definition of trailer when it's as you've described.
, be sure the frame is strong enough at the 5th wheel trailer end. When the tow vehicle hits a bump or dip, that movement will transfer to the trailer with added movement and energy due to leverage factor. Also the dolly will have to deal with the suspension movement of the 5th wheel.
, you didn't specify what model of Polaris Ranger you want to tow, but they weigh from 1,000 to 1400 lbs, dry weight.