Can a carb be tuned with an A/F gauge

Yes, IMO, it is a good idea to do. It helps you to understand and see what changes do what and see how it effects your ride in various ways.

Also understand that the day you tune it is all the time is good for because tomorrow's conditions are different and your carb is out of tune again. This is the carb's drawback vs. EFI. But worry not, millions apon millions of cars have run fine for decades like this.

Basic carbs like the OE factory units and aftermarket (basic) Holley and Edelbrock carbs will only get so good. Upper level (read expensive and more complicated) Holley's have much more adjustments to them and can be as fine as a FI system. But only for that run since conditions change later in the day and throw it off tune.

A dual sensor is good if the device can average in the 2 sides.
The 02 sensors are installed in the headers collector, not the reducer which bolts onto the header or like said above, in the exhausts down pipe. AKA right after the exhaust manifold.