Dear Bill,
Quite a few bikes just had one disc fitted, a Norton Commando is one that springs to mind. My early Commando had the drum front brake but I borrowed my brothers Interstate with a single disc and it didn't seem to be a problem on that.
A Comet being lighter than a twin is probably OK with a single disc. Someone in the Coventry section has a single disc Comet and is happy with it.
Another reason to fit one rather than two disc is that you don't have the problem of having to arrange something to mount the speedometer drive. This was probably the most problematic thing to do when I fitted the twin discs . It is easier if you have a bit of fabrication/machining skill.