The entered IPs will be treated in a “failover” mode (you could have there more than two IPs).
If you like something more fancy you may try to define a subdomain in your domain DNS zone (like
relays.domain.org) and define MX records for this subdomain pointing to your smart hosts (all with same priority => you will achieve DNS “fake” round-robin, other priorities => you will configure the failover rule).
In the end please use this subdomain as your smarthost as Axigen will try to deliver the messages via its MX record(s) (if not found, it will try to deliver via its A record(s), but this will be not your case).
Hint: increase log level to “Protocol Communication” for SMTP-Sending, Processing, DNR and SMTP-Receiving services so you could follow (via the Axigen log file) a test message through all of the mentioned modules.