Diode Selection for 12v motorcycle lighting application
Connect a diode from the 12V side of the aux lights to the 12V side of the headlight.  That way the headlight will still be powered when the aux lights are on.  Actually they will be powered just a little less.  A diode drops around a Volt in this case, so they will get a little dimmer, but not a lot.  This should be more than offset by the aux lights coming on.

You need a diode that is rated for whatever current your headlight draws.  I am not familiar enough with motorcycles to know what typical headlight current is, but I expect a few Amps at least.  You can figure this out from the light wattage rating.  Divide that by 12 to get a rough idea of current in Amps.  For example, if the headlight is "100 Watt", then this tells you the current is about 8.3 Amps.  There is some slop in this, so in this case you want a diode rated for at least 10 A.

Diodes also have a voltage rating, but in this case that shouldn't be much of a issue because 12 V is rather low.  A "20 V" diode should be OK, but I'd feel better about anything rated for 30 V or more.  If you can find something called a "Schottky" diode, that will have a little better characteristics but may be hard to find and more expensive if you do.  A ordinary diode is good enough.

The diode has to be installed with the right orientation.  You want the "anode" end at the aux light and the "cathode" end at the headlight.  These are usually not spelled out explicitly like that.  Sometimes the cathode end is marked with a band if the diode is the shape of a cylinder.  Otherwise, there might be a sort of arrow symbol: