First up, pay attention to Jurasic's comment - good info there. The three wires to the light bulb are negative, low beam, and high beam. There's a bunch of ways you can go about wiring up twin lights - depending on a few things. I'm going to assume you want both headlights to have a low beam and a high beam - if you want it the way the sports bikes do it with one light being high beam only, just don't include the low beam wire to the second light.
First up, do you want to be able to easily restore the lights to the standard configuration? If so you'll need a plug that fits into the standard light socket (you can just use 3 male spade connectors just as easily, just make sure you get them in the right wires), with two wires coming from each terminal, each wire going to the matching terminals on two separate light sockets. Use electrolytic grease on all the sockets to waterproof them. (I'm not being very clear here am I? I'll draw up a picture to go with it. Imagine a double-adapter, that's basically what you're making.)
If you're not interested in being able to easily restore the bike to original and know how to solder, cut the original socket off and wire up two new sockets the same way as above except with the joints soldered and covered in heat-shrink tubing. That's the way I'd do it - it's a lot more reliable
if you can solder reliably.
You could also use "scotchlocks" to make the connections. These are little plastic clamps that have a slotted blade inside them. You lay your new wire next to the original, place the scotchlock over both wires, and close it with pliers so the slotted blade cuts through the insulation and connects the wires. They're simple to use but they score the conductor, which encourages it to break with vibration. I would strongly discourage anyone use them for pretty much anything.
A few tips -
- Make sure the wire you choose is rated for the current it'll be carrying. GO to an auto parts store and tell them you need wire suitable for a 100 watt light.
- You're probably not going to be able to find a plug that'll match the headlight socket (I doubt such a thing exists) so if you want it restorable you're going to need male spade connectors. These things come in several sizes, make sure you get the right size for the wire you'll be using. Generally they're simply crimped onto the wire which works fine as long as you use a real crimping tool rather than pliers. Auto shops sell them for a few bucks, and often you can get the crimpers along with a few dozen assorted connectors cheap enough. You can also solder these onto the wire if you want but choose one or the other, don't try to do both.
- The connectors come in bare or insulated - use insulated ones.
- If possible use the same coloured wires as the bike has standard. If not, get three different coloured wires. It's easier to make mistakes if they're all the same colour.
- Auto shops also sell black sleeving - get a foot or two of this to put the wires in. It makes it look a lot more professional and will help protect the wires against abrasion.
- Insulate everything. Put dilectric grease on everything.
