I rewired the headlight on my '99 380 EXC, but not to run both beams at the same time. Here's what I did; it may or may not be the same on the newer bikes:
The headlight is connected to the electrical system with a...connector. (Imagine that!) There are three wires going from the headlight to that connector: hi, lo, & ground. There are only two wires on the bike side of that connector: ground, and the wire going to the switch. Simply buy an appropriate switch from Radio Shack or wherever, replace the stock one, and also buy the parts to make the bike side of the connector have three wires. Wire it up and you're done. I bought a switch that allows hi beam, lo beam, or off, but I'm sure you could figure out a way to run both at the same time if you want.
I smashed the $4 switch when I endoed once, but other than that it's been absolutely reliable.
Burt