You are very right thats what the pro800 turbo does, But This is a new service that I am getting ready to beta test for nexland and I am told that with this service Tzo.com along with a frimware update for the router will "bind" both IP's here's a link to more info:
http://www.tzo.com/MainPagePartners/NexlandTurbo.html
I've been working on tring to get two PPoE contions bound for some time and have worked with many companys on the matter, Untill now you had to have your ISP bind them at there end, so this is truly a breakthrough. I know this do to the fact I am a nexland dealer.