3.5mm jack signal splitter
Depending on how much money and how portable the system can be, the splitter can be a very good solution.

The splitter only parallels the headphones, there is nothing fancy happening inside of it. This can have at least two effects:


lower the maximum volume
damage the phone


The maximum volume will be lowered because the amplifier inside the phone needs to drive 2x headphones, so they will need to share the power. Whether this is acceptable or not depends entirely on the amplifier in the phone, and on the "efficiency" of your headphones, i.e. the dB per watt they can output.

Damaging the phone is a very remote and unlikely scenario. The amplifier inside is designed to drive a certain load, and you are demanding more. This can potentially overheat the chip, drain the battery faster, or even damage the chip. Again, this possiblity is extremely remote: the chip is most probably protected from short circuits, so it is impossible to damage it.

One alternative you can explore is to use bluetooth headphones. Some phones allow for multiple headphones to be connected at the same time.

Another option would be to use an active splitter; I don't even know if such a thing exists, but it would require an additional battery/power source.

Since you say that your electronics knowledge is limited, I strongly advise against assembling your own circuit. There is no way you can do better than a mass produced, engineered product.

All in all, your best bet is the splitter.