Multiple mufflers

Typical dual exhaust / dual mufflers, exit out the back.

Say I add a Y-pipe on each side, and add one more identical
muffler per side (so 4 total), and so now have 4 tailpipes.

Will the new exhaust system have less backpressure and be louder,
or will it have less backpressure and be quieter ?

That was most famously tested by Jimmy Addison on the Silver Bullet; he ran 4 Caddy mufflers set up as you describe, claimed it was quieter & flowed more. Seems to make sense to me. There's a Youtube clip of the car nowadays but I don't recall is if still has that muffler setup.

Do it and let us know
AKA, what diameter is your pipes from the manifolds or header collectors to the muffs and on back to the second set of muffs?
I think of exhaust systems like electric circuit, resistors in electric circuit in series restricts the flow, resistors in parallel don't increase resistance to the flow
I think four muffs ran in parallel mounted at the rear bumper off of two Y pipes would flow the most and be the least noisy

I'd guess it would be quieter with less backpressure, the trade off is the extra weight. Likely you wouldn't see a performance increase if your system is already capable of handling your engine output.

I don't really feel it's necessary these days with all the muffler options and crossover pipes. An X-pipe and good set of mufflers can be pretty quiet, especially if it has tailpipes.

My 9 second DartSport had 3.5"pipes, X-pipe, Magnaflow stainless race mufflers and the pipes exited on the side right in front of the rear tires.
People would tell me they thought I coasted into the staging lanes because they couldn't hear it.

My current mid 10 sec street car has 3" pipes, H-pipe, Jones stainless straight-through mufflers, and 2.5" tailpipes.
It is very quiet and I love the sound. But I do have electric cutouts for fun and track time. I haven't made a pass yet with the cutouts closed.

I think it would be louder and heavier. I tried to make my 590 cube 1471 blown hemi challenger quiet when I drove on the street. I bought the biggest mufflers I could find to make it happen. I bought a pair of stacks for a semi 4 foot long, 5 inch pipe, chopped a side flat and tucked them under my car. Certainly they should make it quiet? It sounded good and flowed good I'm sure but it was not quiet. I live hear in Detroit and have had people say they heard it from 2 miles away when I jumped on it, the idle set off car alarms all over. I think you would have double the sound. jmo

I was shocked at how quiet my car was when I first put on the 4" Hooker Max Flow mufflers. They are straight through, like a Dynomax Ultraflow, but are real quiet at idle and part throttle. I have an H pipe and dumps at the axle. Have not done a back to back test yet, but it's run real close to what it has w/ just a pair of 4" race bullets on the collectors.

Sounds like a good setup Chip, let us know how the back to back test goes if you get time to do one.

Tailpipes ending where? All the way to the bumper, behind the tire...or...??

CudaDoug, my 2.5" tailpipes are one inch past the back bumper with 3.5" exhaust tips.

Could just run the mufflers and also just install some bullet mufflers on the same pipe, say the regular mufflers in the stock location and the bullets right before the tips.

I was thinking of running 2.5" pipes out from my 3" mufflers to the bumper. I have 3.5" collectors to 3" pipes to 3" 2 chamber old Flowmasters. I have a new set of GIANT Dynomax super turbos that I am going to install this year.
I really don't think the restriction would be that bad after the mufflers
Back to the OP's question I think a small bullet resonator before the tailpipe would take the snap out of the exhaust note

Gus