A stack serves it purpose to get the exhaust out of the truck, and it keeps many tracks and pulling orgs happy. Dual stacks are the same a duals out the back - it's somewhat for show, but still keeps tracks and orgs happy.
I agree with TurboBuick that dual tips on any vehicle (gas or diesel) that come from a splitter are fake and don't do anything - they are just for show. If you have true duals front to back then it's really there to serve a purpose. Not knocking you for wanting the "show" of duals though - my 6" polished tip on the end of a 5" exhaust is just as much for show as duals are, so to me it is merely preference as long as performance is the same. If your dual kit flows less than the same size single kit from that brand you may want to rethink it though.