I have two trucks with sealed beam headlights and both of them use either the high or low beams, never both. If I modified them to use both, would anything break?

Not good idea at all...

The headlight assembly is limited by its design on how much heat it can handle to the power of the one single filament at a time. So by powering both at the same time you get almost double the heat power the thing is designed for, so it will be overheating, damaging not only the bulb itself (the seals and the internal bulb, as I'm guessing it is a halogen, will be exposed to higher temperatures than designed for), but also its holder and the things around.
It may not be immediate damage, but over time it may lead to plastic parts becoming way too brittle and fall apart, paint pealing of the metal and the metal then corroding, sockets and wire insulation becoming brittle, crumbling away and leading to wires becoming bare and prone to short circuits and corrosion...


Cars which use separate headlamp for low and separate for high beam (like the ones using H7 or so) usually keep the low beam ON when the high beam is turned ON, the high beam optics is then designed to complement the low beam, so to throw the light only where the low beam is dark, so reaching higher total light levels. But these cars are designed with this, so the temperature management design of the headlamp assemblies includes this operation.


The combined headlights, like dual filament sealed beam or H4 can not handle the heat and need the low beam to be shut down when the High beam is turned ON.
Only the "warning flash" (aka "light horn") function (when you flash the high beam by pulling the control stick) allows to turn ON both at the same time, but then it is expected to last for just few seconds, so not enough time for the thing to warm up above the design temperatures.

to add to the above

My car uses the same setup with H4 lamps (fairly standard in European cars)

When i am in low beam and suddenly do need the extra light (when i see in the dark in the distance something that might be obstruction on the road) i just hold the handle in the "flash" position untill i pass the place of question (just before the point where it clicks to the "high" position and turns the low beams off). The 10 or 20 seconds won't do anything

If held in this position permanently, for one this would probably melt the headlight reflectors, which in most modern cars are made of nickel plated plastic. In some cars it melts already with the normal setup. (Once it started melting, the plating cracks and bad spot starts absorbing more IR and will keep melting)

High beam for cars should be banned!, it’s just allows people to drive faster at night! 🤨

My truck uses separated HB3 And HB4 lamps for highs and lows. So I could theoretically get away with modding the wiring to run all 4 with highs. which I have already done once. I found that the low beams being on cast too much foreground light which limited long distance viewing. So I reversed the modification and decided to add auxiliary high beam lamps instead. currently I am running 70 watt low beams, 50 watt fog lights, 100 watt high beams, and 100 watt air plane landing lights. 400 watts of high beam is plenty. however I have 250 watt aircraft lights I am planning on implementing. not because I need them. but because I have a crate of 24 I got for cheap. I cannot get a pair of 100 watt lamps for less than 40 bucks.

As long as there are no plastic parts or some small enclosed space inside the engine bay subjected to the heat of both lamps at the same time, there is no thermal problem

Next question is there an electrical problem ? Such as a shared wire providing either + or - to both the high and low circuits (this may be either from fuse to lamp, fuse to relays, relays to ground or anything else) ?

In this case you will have to separate one of the beams wiring completely to a new circuit, with new wiring all the way, new fuse (separate wiring and fuses for right and left beams if there is a condition when you drive and this is the only light on), new set of relays controlled from the original circuit

BTW, something i have seen (at least in a 90s Jap car where i fixed a problem with that) The gauge of the wiring used for the headlight circuits is thin enough to affect the brightness. After rewiring it direct from the battery (separate fuses and wires for right and left, 4 relays) with 4mm2 (approx 12#) the lights were noticably brighter and whiter light

Assuming there is no heat problem, you need to check the electrical power budget in the car. Don't forget the alternator is able to deliver half of its rating on engine idle and that should still be able to cover the steady loads (lighting, engine run and the related pumps, all the blowers,...), otherwise you will kill the battery in a short time...