Javi,
There is 10dB “natural attenuation” when you switch a transmitting source from 900MHz to 1800MHz (cf. free space propagation law). On top of that, keep inmind that a typical mobile station will emitt at a power of 30dB in 1800MHz and 33dB in 900MHz.
Therefore, in a dual band cell, in the uplink path, there is a loss of 13dBm (theory) between 900MHz and 1800MHz zones. In downlink, there is a 10dB loss (the BTS is still emitting at the same power).