I don’t think you understand how suppressors work. They don’t slow the bullet, they slow the escaping gases. What report we have left remains because we didn’t fully scrub the gas velocity as slow as it could be (aka, strip it all from the bullet and have it creep out of the barrel/can at near-zero speed). Swing a rope in the air, you can hear it, even though it is much, much slower than super sonic.
A hat on a hat, in this case, is effectively like using a longer can with more baffles. I personally wouldn’t spend the money to do it, but if I already had a 45cal can, and had a whim to build integrally suppressed, you can be sure there would be a day that I screwed both together shortly thereafter. But I am very hard pressed to buy integrally suppressed barrels. In this case, balancing the reciprocating action might be even more complicated than usual, since you’re adding even greater restriction than a single/conventional suppressor.
Effectively, there are multiple modular suppressors out there which do what you’re describing. If a shooter wants a certain length, they can sacrifice some suppression and achieve the length. If they want greater suppression, they have the option to add length. SiCo Maxim is even an example of a modular integrally suppressed option. The “short” configuration is pretty quiet - hearing safe with 147’s - but the “long” configuration is even quieter. The SiCo Omega 36M and Switchback 22, and the Area419 Maverick all work this way - Add length when you want the greatest suppression, add volume when you want to hit a target length. All you’re doing is adding expansion chamber capacity.
I’d be more tempted to simply buy a longer integrally suppressed barrel if I wanted even more suppression.