i thought you were talking about co-location on the same mast ! ok if both sites are using exactly the same frequencies, then you must ensure that the RxLev(GSM900) falls below a certain threshold so it doesn’t affect the quality of the UMTS (Ec/No). What is the Ec/No limit for UMTS ?
Then there is no perfect answer : you need to limit the coverage of your GSM900 cell, with downtilt, and/or lower height of antenna, and/or power reduction, etc, so that the RxLev(GSM900) is below the threshold at a certain distance.
It depends on radio coverage, basically.
In a typical urban area, with good site design, it is quite safe to say that after 3 or 4km, the GSM site doesn’t provide any coverage (rxlev < -110dBm)