Hi!
I had the same problem, and I found a solution:
In config, don't use the host name as filename for the certificate/key, but use the service name (like "slapd"). Then (the confusing part), store the server's certificate in that "slapd" file. So even if the file name is the same on every server, the file's contents are different.
I use (SLES11, your paths may vary):
olcTLSCertificateFile: /etc/ssl/servercerts/slapd.pem
olcTLSCertificateKeyFile: /etc/ssl/private/slapd.key
olcTLSCACertificatePath: /etc/ssl/certs
Here that works fine, but I feel documentation should talk about that also.