Unfortunately no. The first attenuator in the series needs to be rated for the 50W input power. Otherwise the attenuator will be damaged. Depending on the attenuation value of the first, the second attenuator can have a rating of less than 50W.
For example, if you have a 50W signal and need a total reduction of 6 dB, you can use a 50W 3dB attenuator at the input and then a 25W 3dB attenuator after to the 50W attenuator.