What bulbs are you talking about? The stock headlight housing is for 9007 bulbs which contain the high and low beam in same bulb. If you attempt to power both filaments at once, it will burn a typical incandescent bulb out rapidly because it runs too hot, as well as melt the housing.
If you're sure your replacement bulbs can handle this, there are a couple of options that are easier. One is put a diode between the high and low beam wires so high supplies power to the low but not the other way around. Ideally you'd choose a schottky diode due to that type having a lower forward voltage drop, and it will probably need a modest sized heatsink.
The next step up in complexity that avoids having the forward drop loss of a diode, is to tap off the high beam wire, run that to a relay coil, other end of relay coil to ground, then on the relay switched contacts, you take the normally open side, run the high beam wire to that, then run a wire from the other contact to the low beam wire, so when the high beams are on, the relay energizes, closing the relay contacts so the power from it goes to the low beam.
There is NO modification you can make that is legal, besides DOT compliant same-type incan bulbs. It's all-inclusive. Granted, the verbiage may vary by state and local police, may not have the skill set to do anything but it's still, contrary to good will and not needed.
Jumping to the end conclusion, the goal is not to have the most light possible. It blinds other drivers and if you feel it's needed, you might have a clouded headlight lens but otherwise, your eyesight is too poor to drive a brick on wheels at the speeds you want to go which would necessitate extremes in lighting mods.
If you want to see better driving at excessive speed then you need custom made projector housings and HID setup. There is no cheap/easy way to get that done, it's not worth the bother for a 20 year old vehicle that drives like a top heavy brick on wheels, you should never be going fast enough to need it if your headlight lenses aren't clouded up from UV exposure and if they are, that is the thing to fix.