This is because the MS cannot listen to the two timeslots simultaneosly (CCCH = PCH + RACH + AGCH). It is the same if you have PBCCH the GPRS attached MS listens to the PBCCH not to normal BCCH.
Actually it’s like this:
“Creation of additional BCCH+CCCH is possible but only on the timeslots 2,4 and 6 of C0. The info about the used control channel configuration is sent in the Parameter ‘CCCH_CONF’, which is part of the IE ‘Control Channel description’ sent in the SYS_INFO3 on the BCCH. If more than one BCCH is created for a cell the MSs observe the BCCH on timeslot 0 first and – having detected that there are more than one (from the CCCH_CONF) – select one BCCH/CCCH timeslot for all their CCCH activities on the basis of their CCCH_GROUP. The CCCH_GROUP is calculated from the last three digits of the IMSI and the CCCH configuration”