The confusing thing about decibels is not the Watts versus Volts thing.

It is the following.

If you mix two identical signals (same shape and amplitude) which are in phase, you double the voltage, and so quadruple the power, which is +6 dB.

But if you mix two unrelated signals which are about the same in amplitude, their power levels merely add, doubling the power: +3 dB.

That has nothing to do with decibels at all, that's the fundamental physics (or mathematics if you want) of adding waves, i.e. interference.

If they're identical and in phase, the addition is obviously a multiplication by 2 -> 6dB.

If they're unrelated the signals can also cancel out. So not 6bB. If not 6 dB then what is it? An integral that has already been solved for us :D