The overall load (impedance) when connecting two headphones through a Y-splitter can be calculated by multiplying each headphone’s impedance and dividing by the sum of the impedances. For example, when connecting 300 ohms and 600 ohms in parallel, the overall load is (300 x 600) / (300 + 600) = 200 ohms.
The typical wattage output from the headphone jack for high-impedance headphones is around 3 RMS volts. Therefore, the current drawn from the amplifier is approximately I = V / R = 3 / 200 = 0.015 A, and the required wattage to drive the headphones in parallel is VI = 3 x 0.015 = 0.045 W or 45 mW. This required wattage is higher than the wattage needed to drive the 300-ohm or 600-ohm headphones individually (30 mW or 15 mW). This is obvious because the wattage required for the headphone in parallel is simply the sum. Hope this helps.