My experience below is probably unrelated to this topic, but since a 2-in/2-out muffler is referred-to, I'll post my experience with a stainless steel example I have on my 54 1/2 ton Suburban.
I have Patricks's SS dual exhaust system with a 1960 261 engine. The dual-in/dual-out muffler if mounted about 1/3 of the length rearward, and the tail-pipes "exit" over/under each-other behind the driver's side rear wheel, where the original Suburban (and, maybe all "panel-body") tail pipes exited from under the body.
This system is beautiful, and it was easy to install (well-designed/made). However, it is/was made for boys/motorcycllsts who get-off hearing loud exhaust noise. It is noisy inside and outside the Suburban, at all speeds and when "backing off". I'll eventually try to find a mufller/exhaust shop who will replace this muffler with two original-style "straight-through" mufflers.
Long ago, I had a reproduction Fenton dual-exhaust system from Jim Carter, using two original style/design mufflers and two tail pipes. It produced a mellow sound and was no louder (for occupants in the Suburban) than an original single muffler.
