There's nothing in OpenLDAP that would prevent this. This is a question
more suited to either the pam_ldap or nss_ldap mailing lists. The only
problem is you might have cn=userA representing two different users in
both domains at once, and you'll have to have some kind of policy for
dealing with those situations.
This is really the kind of thing that Samba and winbind does best.
Winbind understands the topology, and creates accounts like AD1\userA
and AD2\userB so that there is no possibility of conflict.