Your HA configuration should be defined as follows:
Server A
Route 0
receive port 25
archive
forward server B 8091
Route 1
receive port 8091
archive
Server B
Route 0
receive port 25
archive
forward server A 8091
Route 1
receive port 8091
archive
To verify whether or not your configuration is working, it may be useful to clear out your SMTP queues.
1) stop both servers
2) delete everything under /var/opt/mailarchiva/ROOT/queue/smtpclient/* on both sides
3) start both servers