I am also operating dual-band network(1800/900) and found that SDCCH usage at overlaid subcell is very low. Actually, SDCCH traffic exist, but very small.
The only explanation, which I can imagine – is that 900 cell signal strength is much bigger, than 1800.
Usualy, I have SDCCH/8(NCOMB BCCH MODE) at underlaid 1800 and SDCCH/8 at 900 overlaid cell.
So usually – all SDCCH traffic comes first to 1800 cell, then to 900. If you have no overload of SDCCH traffic at 900 cell – you will not see traffic at 1800 as well.
But SDCCH traffic at 1800 exist, if you have no IDLE SDCCH at 900 cell.