Incoming connections cannot be load-balanced, they will come on a WAN interface. What you MUST do is make sure the reply goes out of the SAME WAN interface, otherwise your peer will drop it. Although this should work by itself using the NAT table, you have to do it manually.
For example if you have a web server 192.168.1.2:80 and a mail server 192.168.1.3:25 with ppp0 and ppp1, then you have to make a netbalancer rule for 192.168.1.2 s_port 80 to go through ppp0 and create a virtual server on ppp0 for d_port 80 requests. Same for mail on the other WAN, create a netbalancer rule for 192.168.1.3 s_port 25 to go through ppp1 and a virtual server on ppp1 for port 25.