Actually GSM mobile max power is +33dBm, but basestation power is greater than +35dBm, I guess you mixed up here.
Say basestation power is +50dBm in peak, and GSM has some pilot channel, other system are same, so the attenuator max input is +50dBm+10log(12*2)-15.1+3dB(redundancy)=51.7dBm=21.7dBw. This is huge.
In reality, when multi-system, some TX may be back off from max power, but it is really hard to evaluate it.
The cost way is using a standalone attenuator and coupler for each channel, then is +50dBm+10log2-15.1+3db=40.9dBm.