same BCCH or adjacent BCCH will lead to very poor RF performances, whatever the BSIC value. co bcch and adj bcch should be located as far away as possible from each other.
(adj BCCH, same BSIC) will not lead to “phantom” allocation like you are seeing with (same bcch, same bsic). Because the frequencies are different.
(same BCCH, adj BSIC) might lead to some problems, because actually the BSIC is not acting as a filter. The two BCCH will understand each others Channel Requests. I might be wrong though, let’s say I’m 80% sure about that.