Stef,
The certificates used in PEM are defined syntactically by
X.509, so it's not an issue of what a PEM certificate will allow, but
rather what an X.509 certificate will allow. So, you don't have to
read PEM RFCs to find the answer, just X.509. I'll save you the time;
in an X.509 certificate there is exactly one DN allocated to identify
the subject. One could store multiple certificates in a directory
entry. If a Canadian wanted to have two certificates, e.g., with the
same key, with French and English name variants, that would work,
irrespective of how they are stored in the directory. Alternatively,
use of one DN choice and a alias for the other would probably satisfy
most folks (and provide a subtle way of expressing a linguistic
preference?).
Steve