1 SDCCH ts must be located on the BCCH carrier, so that if only 1 TRX remains alive (all others = dead), it would contain all necessary ts for call management (BCCH + SDCCH + TCH)
(last TRX alive would be the BCCH TRX… remapped dynamically on the live HW)
Other SDCCH can be allocated on other TRX, on whichever TS you need. The recommendation is : “at least 1 SDCCH ts on the BCCH carrier”.
Not “ALL sdcch ts on the BCCH carrier”.