I’m sorry but my vendor is Alcatel so I can’t give you a exact action plan.
I can tell you how Alcatel ideology is organized.
At BSC level we have a “preffered band” ( in my case it’s DCS ).
This means that when my serving cell is from GSM band the mobile can go to any cell.
But … when the serving cell have a BCCH from the DCS band some HO reasons are inhibited in such way to keep the traffic in the preffered band. It’s very hard to go to “not preffered band” if we don’t enable a specific parameter at cell level.
May be you ( Siemens ) have something similar.
From radio wave propagation point of view it is always eazy to capture traffic on GSM band.
What are MAX Tx POWERs ( BTS GSM and BTS DCS )?
You can’t establish MOC and MTC in the GSM cell or MOC and MTC are established on GSM but there are a outgoing HOs to the DCS cells?