In ericsson, all the signalling(BCCH, SDCCH, PAGING, FACCH, SACCH) go through separate timeslots on the abis. That is why concentration is done. In that sense, it would be incorrect to assume that each TRX uses 2 abis timeslots.
In my network, each site is typically 4/4/4. We have 2 EDGE timeslots per cell,2 SDCCHs per cell and a CONFACTOR of 4. Yet we use only one E1 per site.
Per NJ’s calculation, this would be impossible.