BDB1 SERVICE BULLETIN
I haven't had a chance to check it out yet but here's my theory.
I have design experience using LEDs see IntelliDash-
Remark Industries :: Performance Digital Gauges & Instrumentation
Super bright high intensity RED LED (T1 3/4) have smaller light emitting viewing angle cones. These RED LEDs could be a 15 degree VA cone. A 15 degree cone delivers higher intensity than a 30 degree cone. Mine look the same intensity when viewed 3ft behind my bike.
I think it's possible that the tailights are not equally tilted vertically causing the viewing angle to be out of range when you are following behind. Don't rule out a horizontal misalignment as well.
I want to initially check BDB2's taillight housings using a tilt angle gauge on the housing face after the bike is plum in the lift.
If the housings are identical then I will look at the circuit board mounting.
Easiest thing to do is adjust the taillight tilt until they seem the same intensity while viewed from 50ft back.
If it's not the above causing the difference then it's probably a current/voltage drop through the harness to the one side.
Let's face it, we're all a little dim in one area or another-