It's probably out there, but all you'd really have to do is get a high-powered diode and wire it from the high beam to the low beam wire. It'll have to handle at leat 15 - 20 amps. One each side and you're done.
Of course, a diode is a electronic device that allows electricity to flow only one direction. If you just connect the high and low beam wires together, they'll both be on at the same time.
Also, due to the intense heat of this mod, you'll probably burn out the bulbs in 1-2 months and then have to replace them. It's probably cheaper and easier just to get brighter H4 bulbs.