Some use this solution with halogen spots instead of the original headlight.
I know one used 3X20W with a narrow beam, 8 degrees, to get the light there he wanted.
However, depending on weather, dust condition etc. you can use different solutions. For enduro I put my vote on helmet lights. If it is raining or it is a lot of dust a good light on the bike could be better but otherwise, helmet light is more important. If you haven’t tried it I think you really should.
When using halogen spots, only use the top of the line units. It's a big difference. On the paper Philips MasterLine ES is the best. The name of the Osram top of the line model I cannot remember. They are two-three times more expensive than the other but it is still no big deal. In Sweden it is less than 11 Euro for one unit. You shall use the one with 51mm reflector/lens. In my gallery you will find a cheap helmet light solution many uses.
Put the two yellow wires together or not. I will say it depends on.
If you put them together and connect everything to the battery you will lose around 10 W due to the conversion from AC to DC. On the other hand this is the only solution for HID. I haven’t checked this; does the bike run with both the yellow wires unconnected? If so, measure the voltage between the wires. If it’s not zero you will lose power if you put them together.
When my bike is running again I will do like this. I will make a tail light made of “light diodes”. Buying one is probably a better solution but it’s funny to do things by your own. I will put the original headlight over the battery. I have the 55/60 W model. The power to the helmet light I will take from the AC regulator. You probably will use more than the alternator gives the battery but then the headlight starts to fade away you just switch it of and only use the helmet light for a while. I promise you, the helmet light is enough if you don’t have power for a headlight too. If you only will go off-road, replace the headlight with halogen spots or take a reflector from a car and fit it to the bike.
If you will do a lot of off-road riding in the dark the HID is the outstanding solution. Anders Eriksson uses three units. Two on the bike and one on the helmet. All driven by separate batteries. The original headlight he also uses. See his photos,
http://www.ae-racing.com/sv/pics.php?id=14. That race was a little wet.