Welcome to the sabre family. Congrats on the new silhouette, may it bring you many happy camping memories. I am no expert on towing tandem but I do know it is illegal in some states. Other states there are length limits that a 20' boat may well put you over. Another consideration is whether or not adding a hitch will void the warranty. Just some starting points for you to look into.
2014 Crew Cab Chevy Silverado 3500 4wd Duramax/Allison
2014 Sabre 34REQS-6