On the XR600 setup, I ran one spot and one flood. If I were to do it again in the VERY tight woods I would run dual flood lights.
I took out the XR light in the shell there. Took a plastic trash can, cut a piece that would fit in its place. Then used a speaker speed nut on the back of the stock headlight screw to hold the unit to the shell. I found a few $.97 flash lights and used the ring that screws into the body to hold the lights. Then just wired them up by Ying the same post on both lights together. It held up great offroad. I have not had a lot time with it as the days are getting longer now. But the lighting was MUCH better on the trails in the twilight hours.
On the KDX250, I have the UFO twinns light:
http://www.ufoplast.com/images/FANAAL.jpg
That has one spot and flood. I still have the other 2 lights from the XR project I did, so I have a spare of each bulb. Again, my KDX is weighing in at 125lbs at the moment (no plastic, tank, airbox, everything to do with the rear suspension and brakes).
Some other notes is that you can get those bulbs in the 2" size in 35w and 55w. The 20w units are like 1.5" in diameter. Check your local home depot for them. They do have a lifetime warranty, so I kept the packages and receipt. Should be interesting if I go through one here. Hopefully it will be a painless swap out.