Lock file conflict with multiple postmasters

Hi,
We have two postmasters on the same host. They are configured to listen on
port 5432 of two distinct IP addresses as follows:

/var/lib/pgsql1/data/postgresql.conf:
listen_addresses = '10.0.0.1'
port = 5432
/var/lib/pgsql2/data/postgresql.conf
listen_addresses = '10.0.0.2'
port = 5432

However, if we start the second instance while the first one is running,
we get the following error:
FATAL: could not open lock file "/tmp/.s.PGSQL.5432.lock": Permission
denied

So, lock filename cares only about port number and nothing else.
In order to avoid this conflict, how can we configure the name of
lockfiles or directory where they are placed?

Thanks in advance!
Nicola