I used the Load Balancing Wizard. The DSL modems are set in bridged mode and I am getting external IPs on each one via DHCP. They are on different network segments.
I've been going through learning at much as I can, but mostly built everything via the gui, been getting my feet wet with editing the config further with the cli though. I think applying the balancer to the guest network is all I needed.
Thanks for your help!