you need about 30dB decoupling between the two systems (GSM900 and UMTS). As far as I know, you can place both antenna systems right next to each other, and the decoupling will still be higher than 30dB.
Out of safety, I’d keep a distance between the bottom of higher antenna and the top of lower antenna of :
Dv > 50cm
(50cm provide a decoupling of about 50dB)