Hi MKT,
Sorry for being late to post. but you can try having a seperate BCCH for DCS coolocated with the GSM and prefer DCS calls by using C2, CRO and disabling PBGT handovers. Also if you are using SIEMENS then I think there can be allocation to DCS based on TA. similar to TA basedHO for extended cells. Also adding a combine to the DCS would only reduce its coverage pattern.