I’ve experienced plenty of situations where having more than one light has come in handy. The problem is, there’s only so many places where a person can mount lights on a bike and there aren’t enough people riding at night to make light mounting points a way to distinguish one bike from another.
I’m fairly picky about mounting. The cheapest lights don’t always mount well and people who try to install lights don’t always read the manual. The problem is that it needs to be able to clamp to a variety of different sizes of posts and there’s not standardized mounts for things nor standardized handlebar sizes nor anything else.
Most front-facing lights are designed to clamp around the handlebars. I like to have enough comfortable gripping positions on my handlebars and have room for a cyclecomputer and a bell, but that’s tricky with straight mountain bike handlebars. I was able to mount my cyclecomputer to the stem instead of the bars.
I think exactly one Cateye light is able to also mount to the stem.
The best solution I’ve found is putting one light lower on a Minoura Bessa mount. The Bessa hose-clamps around a suitably sized tube and provides a handlebar-diameter mounting point. It, of course, doesn’t work so well with a carbon-fiber fork because it’ll scratch it and eventually cause it to unravel.
I’m also fairly picky about the removability of my front lights. See, most all lights are designed such that you can unclip them all rapidly and toss them in your helmet while parking a bike such that nobody’s going to steal your stuff. But it’s also valuable if you want to do something silly like mount a camera to your handlebars that you can take the whole works all of the way off.
The rear lighting situation is slightly less troublesome. See, there’s fewer pieces of hardware to mount and the lighting needs in the rear are often less critical, because it’s all about being seen and much less about seeing.
You can generally easily mount one rear light underneath the saddle, either by clamping to the seatpost or by clipping to the spares kit in the saddlebag with the fabric clip. You can often mount another rear light on your clothes using the same sort of fabric clip. One problem with the fabric clip is that you can’t always assure that it’s going to dangle in the exact right orientation to orient the light and reflector properly.
There’s often seatstay mountings, but on my bike, the rack gets in the way of the seatstay light.
It turns out that, in Europe, there is a standard mounting for bike lights on a rack… pretty much two screw holes of a particular size a particular distance apart. My rack has these holes, plus an alternative set of holes, but apparently many American bike racks either only have one set of holes or have no holes at all. The problem, of course, is that the European-centric holes are where the button on an American-centric mount would be.
Some lights don’t come with appropriate rack mounting bits, although you can often purchase the bits directly from the maker.