coordinating 2 amavisd/spamassassin scans -- 1 PRE-queue, 1 POST-queue ?

[PGNd]

I run

        postfix 3.0.1
        amavisd-new-2.10.1

I've setup a postfix to use both pre- and post-queue filters.

I' configured amavisd to provide two spamassassin scans, one in each of two 
policybanks.

The 1st to do 'lighter' pre-queue SA (headers, no bayes) (and other) scans, the 
2nd to do 'heavier' SA-only (body + bayes) post-queue work.

The two policy banks use different SA configs

        $sa_configpath = '/var/lib/spamassassin';
        $interface_policy{'10001'} = 'PREQ';
        $policy_bank{'PREQ'} = {
          protocol => 'SMTP',
          originating => 0,
          enable_dkim_verification  => 1,
          enable_dkim_signing       => 0,
          bypass_decode_parts       => 0,
          bypass_spam_checks_maps   => [0],
          bypass_banned_checks_maps => [0],
          bypass_header_checks_maps => [0],
          bypass_virus_checks_maps  => [0],
          ...
          sa_siteconfigpath         => '/etc/spamassassin/local/prequeue
          sa_spam_subject_tag       => ...,
          sa_tag_level_deflt        => ...,
          sa_tag2_level_deflt       => ...,
          ...
        }

        $interface_policy{'10002'} = 'POSTQ';
        $policy_bank{'POSTQUEUE'} = {
          protocol => 'SMTP',
          originating => 0,
          enable_dkim_verification  => 0,
          enable_dkim_signing       => 0,
          bypass_decode_parts       => 0,
          bypass_spam_checks_maps   => [0],
          bypass_banned_checks_maps => [1],
          bypass_header_checks_maps => [1],
          bypass_virus_checks_maps  => [1],
                ...
          ...
          sa_siteconfigpath         => '/etc/spamassassin/local/postqueue
          sa_spam_subject_tag       => ...,
          sa_tag_level_deflt        => ...,
          sa_tag2_level_deflt       => ...,
          ...
        }



Question:

With those two policybanks, is

        $sa_num_instances = 1;
        @spam_scanners = 
(['SpamAssassin','Amavis::SpamControl::SpamAssassin'],);

still sufficient config?

Or must I increase/modify, e.g., eith either/or

        $sa_num_instances = 2;
        @spam_scanners = (['SpamAssassin1','Amavis::SpamControl::SpamAssassin'],
                      ['SpamAssassin2','Amavis::SpamControl::SpamAssassin'],);

?



Question:

Also each SA scan -- prequeue & postqueue -- can add SA headers.

Should both do so, using different header names?  If 2 sets of headers, is 
there any 'header cooperation' (making the 2nd scan aware of the 1st scans 
headers) that needs to be orchestrated between then 2 scans, for either 
functional or performance reasons?