The reason for the single stack rather than the double is that the double stack costs hp because you are spliting the air and making unwanted back pressure. Otherwise 2 actually looks better. JMO

FYI, a guy running a T (not a Y) on a 1000rwhp truck found 60 hp just taking the T out and running a single. Where else can you pick up 6% that easy? It would be less with a Y, but the concept is the same.