Instead of doing a double-Y internal muffler, just use Magnaflow 3" by 18" dual resonators. They will fit and achieve the same thing with way more flow.
Setup below is two magnaflow 3" x 18" resonators, corsa sport, catless mids, 2" longtubes. Mild startup, tame cruise, wrath of hades WOT.
Cold start vid:
https://youtu.be/HlQc12MiO18