Hello Andy,
Power of DRU is 46 dBm ( 40 W) for GSM 900 and 45.5 ( 35 W) for GSM 1800 .
In Combined mode ( 2 DRUs used in the cell which gives 4 TRXs ) you will loose 3.5 dB. The u will got : 46 – 3.5 = 42.5 dBm for GSM 900
45.5 – 3.5 = 42 dBm for GSM 1800.
Regards