My 1997 manual says 200W. I don't think the 640 is any different. Hey, on the highway, light load, under 75 degrees F, you should be able to run both filaments. The minute that fan kicks on, all bets are off.
As others have mentioned, you can use the 'mid' position on your thumbswitch to run both filaments when you need the extra light. I do the same thing on my ATK off-road (high and low filaments burning in the same bulb).
Nothing like a 7" Hella with both filaments burning to light your way