Question about Samsung Duo

In my experience, this power supply unit can only charge 1 smartphone (S21) OR 1 earbud at a time. Simultaneous charging of two smartphones does not work.

It may be possible to charge a smartwatch at the same time, but I haven't tried it.

Yes, my display on the power supply unit shows up to 20 watts. The charging power depends on the power supply unit. For 15 watts of power, the power supply unit requires a current of 3 amps at an output voltage of 5 volts.