It's about the mass an exhaust can move efficiently and the requirements of the engine it's supporting.
Too big on a NA engine and you loose flow velocity and scavenging effect. To small and you run into restriction.
Single or dual is really irrelevant if the system is sized and designed properly.
A proper single to support the same engine will have a larger diameter than a dual system.
Single systems are better for emissions as the converter gets up to temp and stays the longer. Dual systems can also do just as well if properly designed.
If the emissions laws state you need to be Oem and your vehicle only came with a single it would likely be easier to get by a "sticky" inspector with a single. If the vehicle was available with both then no problems. If a dual on a single has a CARB sticker or the like then no problem.
There's lots of opinions on single vs dual, but ultimately it comes down to personal choice. Performance is a moot agreement because each can be made to perform equally well.
There'll be lots of argument on that point also......