Originally Posted by till
Or you find someone with the same problem and you provide the secondary for the domains vice versa.
I don't play well with others so that's out.
Seriously though, since adding a second IP, in my situation, is about the cost of a Coca-cola, I'd be more inclined to get everything working with two IPs and two NS's and then offer others, on a limited basis, free NS.
Thanks for the suggestion though as it will likely be useful to others reading this!