Actually, the volume may not be reduced at all, and if it is it probably won't be anywhere near 50%.
When you split a signal like this, you're increasing the load on the output amplifier. Depending on the output circuitry, it might be able to drive both loads with the same current, meaning that you might actually get the same volume levels.
Even if the output power were cut in half, that would not result in a 50% drop in volume. You would need to drop the power by a factor of 10 to decrease the volume by 50%.
Generally speaking, you can split a single output like this into two loads without significant signal loss or degradation.