hi,
if bcch trx goes down, then its logical configuration (all the 8 TS) are shifted to another TRX.
So my recommendation is to have 2 SDCCH on the BCCH TRX, and then 1 SDCCH on 2 other TRXs.
The last TRX will have no SDCCH, in order to favour it for PDCH Allocation (the idea is to have 8 consecutive TCH/PDCH on the same TRX)
cheers
pix