Seems like it would be a good idea to define a new option "glue-peer" or somesuch that allows multiple peer-level DBs to be glued together.
http://www.openldap.org/lists/openldap-software/200802/msg00128.html
It wouldn't really be much work in backglue.c if the set of entries is disjoint.
In case of duplicate entries, we'd have to track them and either drop some of them or merge them.