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.
Thanks. I seem to be having trouble finding this mod. I guess I can see why when I think about the stock lights. I have the dual beams on my 96 and want both to light up when I hit the highs. I don't think heat will be that much of an issue inside the bulb. Maybe at the relay or point where the diode will go. Upgraded wiring may be needed for extended periods of operation, but the short burn times I'm thinking of might be okay.
FWIW, my lights already have a relay wired in to fire both beams at low power for a "citylight" feature. It is triggered when I flip the highs on and then dial back off the main light mode. The highs dim and lows light up, all 4 beams look really nice. I should have a pic around here somewhere.