I have the 2 into 1 magnaflow muffler that I got from BC a few years ago.
Looking at its internal construction... it's actually a 1 into 2 (which is much more common) but since the design is a straight pass-through perforated internal with packing... it really doesn't matter that we're all running them backwards.
Now on the sound question? It was fairly quiet and subdued for about two months... now it's as loud as any other pass-through design muffler that's been broken in.
I've got the WH shorty headers, 2-1/2" diameter down pipes into the muffler, and a single 3" diameter pipe out the back.
Sometimes when I'm tired of the volume level screaming down the interstate, I ask myself, "how could I expect it not to be loud!".
Honestly, I do plan to swap out to a quieter muffler at some point when I'm working on exhaust stuff again.