[Dovecot] Multiple postfix + single dovecot - optimal configuration

[Tomek]

Hello,

I want to setup dovecot with two postfix servers on three machines:
- dovecot - with huge amount of disk space - where all user emails will
be kept,
- postfix1 - with greylisting, amavisd-new (spamassassin, clamav, etc),
with mysql backend,
- postfix2 - same as postfix1.
MX priority will be set to the same value for both postfix machines.

For now I don't know how to "glue" these severs:
- if I should install dovecot on both postfix and use third servers as a
NFS share?
- or maybe it is possible to communicate with dovecot on third server
via LMTP?
- or should I run third postfix daemon (without antyspam etc - simple
configuration) on the same machine as dovecot only for delivering local
mails?

I will be grateful for any advice, howto or good practices example.

-- 
Regards,
Thomas.