Dual quads should be setup, that you run the most time on the primary of the first carby. Than with 30% or 50% opening the Primary of the second carb should come in (linkage should be designed like that). So far you are running a double pumper. If you step onto the pedal the two Secondarys come it additional. In the upper RPMs you get more air trough the engine. So the engine should turn much more freely above 5000 RPM.
That is the experience of a friend who is running dual holleys on his 302 roller cam with AFR165cc heads. So he has a healthy engine, as you also do.
Downside is, that he doesn't get idle as low as with one carby. And you need to fine tune the two primarys similar to a double pumper. It helps if you have a manual transmission (which my friend has).
4 barrel (if not way too big) is usually better, than 2 barrel because the primary venturis are smaller. This means you have higher air speed and a better atomization of the fuel, which means better MPG and performance. But you need to tune a universal carb much more to work perfectly. The OEM carb will run nearly perfect, because the OEM set it up. One point: OEM sets it up, that it is running everywhere (altitude, quality of fuel etc), so he has some secruity factor on the stock setup (this is also valid for ingition timing).
Regarding AFR at idle: forget the reading, there is to less mass flow, that the reading will be right. Manifold vacuum is at idle the better way for adjustment. But I understand that you are wondering that it shows a fixed reading. Hope the device has no defect.