yup you are in a fix alright…
Well you can use the cell in that configuration but then you would have to define the cell as single cell and use different FHS’s.
But as the no of such sites increase the noise floor on your GSM would increase, and KPI degrade grately and you would not have that specific control on traffic assignment to different band cells. what is the optimal if you do insist to go with your config. is to use lower power on GSM TRXs or down tilt it severely as compared to DCS and assign lower priority to TRXs of GSM all these would reduce the noise floor of GSM so your KPIs would not be effected to that extent.
I would say nevy_14’s option is better and you can use it only on cells where you have blocking (not even all sectors to have GSM + DCS)add GSM BCCH and one hopping radio, assign fixed specific frequencies in SFH no 1×1 or 1×3 like stuff.