Unfortunately the resources allocation are very vendor dependent… I’m not sure I can help you, but I can describe the main concepts used in Alcatel.
In case of multiband network, the GSM900 is supposed to provide all the GPRS capacity, while the DCS1800 cells just have one PDCH statically defined. The point o this PDCH is that a user camping on DCS1800 (they have bigger C2) will always have a PDCH available. As soon as the TBF is allocated, the user is sent to the GSM900 cell (it is a special feature of the NC2 reselection, called “redirection”)
Now, you want to allocate some more PDCH in the DCS1800. Unfortunately, these are also the cells that carry most of the voice capacity. Basically, you are trying to squeeze both voice and packet within the same cells (ie. the DCS1800). It is risky : the voice being always the favourite, the packet service will be heavily impacted !
A better strategy is too unload as much as you can the voice in the GSM900. Make as room as possible for the PDCHs.
Then activate NC2 and push packet traffic onto the GSM900.
Now, it’s up to you to implement this strategy, I can’t help regarding which parameter you should change to do that.
“redirection” can be achieved by setting a very quick and easy detection of NC2 reselection from DCS1800 to GSM900 only.