If you have to move the loaded trailer by Hand, then the Single is the way to go. On the other hand twin axles allow for the use of smaller tires, allowing the Trailer, and the boat to be lower, making it easier to launch/load. But 2 axles means twice as much maintenance, and twice as many tires to eventually replace, granted each small tire is likely cheaper. 2 axles also offer the the possibility for double the Brake Capacity
On a Tandem, especially one with Torsion Axles, the Hitch Height is Extremely Important, as the Trailer needs to be almost perfectly level to ensure that both axles are carrying the load evenly between them.
Your boat and motor sound fairly Light, and most Tandems start at 4000 lbs. EZLoader has a Single that is speced at 3700lb load