You still want two because lights break.
I think that pretty much trumps any other argument.
Part of any serious, any thoughtful discussion of bicycle lighting systems should be: How much money is left for my backup "emergency" light and or extra "all night" battery power. It does little good to have an exotic 1000 lumen light head if you are constantly running low on battery power.
Two of my strategies for all-night riding are using a low-power light on "strobe" mode until complete darkness so as to spare my main light's battery for really dark roads. Custom made "AA" battery packs serve as additional redundant power as well.
You see: When you're as big a weenie as Richard Cranium, you have to have
TWO types of battery sources as well as
TWO light heads to see where the sun don't shine.