The SDCCH traffic is in that cell where you make the MSs to camp in idle mode. There are different strategies. For example Idle mode on 900 only (you play with cell bar qualify or C2 criteria) and as call starts (so its the SDCCH on 900) capture handover to 1800 (or whatever handover is called by the vendor). The result is no traffic on SDCCH on 1800.
But if you have concentric dualband cell then you cannot have SDCCH on the inner zone (1800)