A GSM network with a cluster size of 4 which is considered giving the best balance between capacity and interference would require 4.8 MHz of spectrum (see
http://xa.yimg.com/kq/groups/2152165...4-phpapp01.doc). For UMTS you need a 5 MHz block.
If you add a guard band between the GSM and UMTS network of 0.2 MHz you end up with exactly 10 MHz.
So you definitley can operate a GSM and a UMTS network concurrently with 10 MHz of spectrum, however capacity would be modest. So they will need to shift as many users as possible to the AWS band in order to leave enough capacity on the 1900MHz band.