you can precisely compute the 1800MHz harmonics. what are your ARFCN of the 900MHz sector?
this happens when the DL frequencies of 2 TRX (or more) of the same sector are combined on the same antenna.
f1(DL) + f2(DL) = 1800 MHz frequency (UL or DL)
The most dangerous harmonic is one which falls within the UL band. Because even a weak signal will probably interfere strongly with the UL from the MS.
to avoid intermodulation in the 900 site, you should avoid combining the Tx signal. in other words, bypass your combiner or find a way to avoid combining these 2 freq together.
another solution : change your 900MHz frequencies so that the harmonics are far away from your 1800MHz band.