I think the magic is in the harley adaptor harness. With the stock single H4 bulb the low has to turn off when the high comes on to keep from overheating the bulb.

 

I need to know what the rectangle piece that comes off of the harness is. I think that is what keeps the low bulb on when the high beams are on. I'm using a modified 2014 harness and it does not have one.

 

I don' have that on mine. It's a direct plug n play from the single to the dual with another line which I believe is for accessory lights. Here is what mine looks like......

 

That's what the one looked like that I wired an H4 end to for use on my 08. My low beam does not stay on when I put the high beam on.

 

Sorry wrong pic......this is the harness that came with my Dual Bulb kit. The part # is 73272-11

 

There is three sets of wires comming from the plug, one for the high and one fro the low. The 3rd set is the "mystery" wires inside of the black insulation that loop abck into the H4 plug. Im assuming there is a Diode inside that jumps the high and low wires keeping the low beam on when the high beam is switched on but keeping the high beam from staying on when the low beam is switched.