The Blackburn 2'Fer is a good helmet mounted light. Very lightweight -- I hardly notice it's on the back of my helmet. Pretty good dispersion considering there's no diffusion lens, and better side visibility than comparable small USB lights I've seen locally.
That Knog Blinder 4 appears more effective than I'd expected. The apparent size of the red patch makes it pop against a busy urban background, which is already cluttered with red lights.
The vertical orientation of stacked lights is a good plan, even if you set this up only for the test. At a glance it's reminiscent of a railroad crossing light or traffic emergency light. And the separation helps drivers more easily judge distance and approach speed.
For the past few months I've been running at least two rear red lights, usually the 2'Fer on the helmet in flashing mode (it lasts longer) and a steady taillight on the rear rack. A new-to-me used bike I just got last week came with a Bell saddle that has a surprisingly effective horizontal row of seven red LEDs. Even though the original protective lens is missing the lights still work. And because these are the older style, slightly less bright deep red LEDs, they're still effective even with a red colored lens. The newer, brighter LEDs are more whitish orange rather than deep red, and need a red lens to have the same appearance. You'll probably notice this quirk with the Blackburn 2'Fer, especially with some digital sensors -- it'll appear nearly white rather than red, although it appears red enough to the human eye.