think about it this way. a 20W 3dB attenuator has a maximum input of 20W. If you put 40W into that, then you'll damage it, that's too much power.
However, at the output of that 3 dB attenuator, there's now 10 W. If you have a 10W attenuator, then you can use it safely there, and stack up attenuation that way.