Combined BCCH means that one the TS0 of the first TRX, instead of one FULL timeslot for BCCH only, it is SHARED.
Combined BCCH (or CBC) consists of half-SDCCH and half-BCCH. In terms of capacity:
Only 3 CCCH blocks (instead of 9 blocks in a full BCCH)
Only 4 SDCCH subchannels (instead of 8 sdcch subchannels in a normal SDCCH timeslot)