Getting the most from Two WAN interfaces

At the moment I have configured a load balancing Router, with Web-proxy and other simple queues to prioritise traffic. untill now things go fine from the inside.
But I wanted to do more, the WAN interfaces are used only by LAN users, how about external?
I mean, I have a FTP server, Mail Server. How would the outsiders get the most using the two public IP?

At the moment I have done NAT rules for incoming trafic only on one of the two WAN interfaces like:

chain=dstnat dst-address=10.10.10.10 protocol=tcp dst-port=25
action=dst-nat to-addresses=192.168.3.4 to-ports=25
chain=dstnat dst-address=10.10.10.10 protocol=tcp dst-port=21
action=dst-nat to-addresses=192.168.3.4 to-ports=21

Is there any way to reach the inside servers, from outside using both WAN IP-s, without giving the IP? make it act like a DNS wich will than resolve once on the first IP and than on the other?

Toni