Well, basically you can aim for the ultimate perfection. So two sensors may seem to be a must on a twin carb engine. But then you are dealing with fifties carbs and their limited scope of fine tuning: Main jet, idle jet, needle, needle jet, slide with its cut out. There is no acceleration device on these so in the end you will opt for a slightly rich setting for max power and no hickup when wringing the twist grip for acceleration. You will soon find out that you had enough of trying various settings and components and accept a good compromise. And for that a single sensor will do fine on a twin carb engine. Or would your target be the highest possible fuel saving like in car manufacturers rolling road tests to be issued for motor journalists ? I did use O 2sensors to sort out puzzling behaviours on some engines after running out of ideas. I cannot tell in all moments if the engine is too lean or too rich while doing a test ride. I had funny effects on a Honda Clubman when I discarded the air filter box and mounted a bell mouth to the rubber diaphragm Keihin: Only after I placed an action cam behind the carb I could watch the slide jumping up and down half an inch at certain power settings - hopeless for road use. The Keihin was dumped and a classic Dellorto went on and with a little help from the O 2 sensor the Clubman does what we expect.
A simple four wire sensor at 20 Pounds will do the job for your orientation towards a nicely going bike. The single sensor thread in the pipe is plugged after testing and is not visible to bystanders normally, and even so, what could be wrong ?
Vic