The BSIC of two cells should be different ONLY if they use the same BCCH.
The TSC shall be equal to the BSIC.
Finally, the impact of co BSIC and co BCCH : SDCCH Assignment failures (and or Random Access failures), HO failures, and possibly wrong decoding of TCH (interference) due to same TSC.
One RACH sent to one cell will also be decoded to the other cells. Both cells will send an Immediate assignment, but only one will receive an answer from MS. The other one will count this as a sdcch assignment failure due to poor radio (alcatel systems).