> Can I achieve a 'merge replication' between two database nodes with
> slony1?  I know that  slony1 is a master-multi slave replication system,
> but, is it possible to have two nodes, both being publisher and
> suscriber of the other?

Simply put: no.

There are plans for Slony-II to support something like that, but not for
Slony-I.

Multimaster replication is fairly likely to require modifications at the
database engine level, which would break the intent that Slony-I be
compatible with multiple versions of PostgreSQL.