Two pairs of headphones on one output. The forum answer is “you lose half the power”, and that is wrong — with a perfect amplifier nothing changes at all. Both pairs see the same voltage they would have seen alone, and both get the same power. Every bit of loss comes from the amplifier's output impedance not being zero.
The real problem is usually somewhere else entirely, and this shows you where.
The low-impedance pair wins. Two headphones in parallel present the amplifier with less than either alone, and the lower one dominates that figure. Put a 16Ω in-ear alongside 300Ω over-ears on a source with 10Ω of output impedance and the over-ears lose over 4 dB while the in-ears barely notice. The pair you were trying to share with gets the worse deal.
Sensitivity is the real killer. Level is set by voltage, and voltage sensitivity is what matters when both pairs see the same voltage. A typical in-ear can be twenty-five decibels louder than a pair of 300Ω over-ears at the same setting — there is no volume position at which both are comfortable. That, not impedance, is why sharing usually fails.
Current adds up. The amplifier supplies the sum of both currents, not the average. A portable source that was comfortable with one pair can hit its limit and clip with two. The power curve shows where that limit sits.
Never split a balanced output. On a balanced or bridged jack both conductors are driven — there is no ground. An ordinary splitter shorts them together and can destroy the amplifier. This applies to 4.4 mm and 2.5 mm outputs, and to any adapter that ties one leg to ground.