[Porter]

only one dns from one isp? how does the internet resolve your names when isp a is down?

I think with your current setup the only way to solve the problem would be to use a autonomous system where all your routers a connected to, your basic problem is that isp a does not route ip range from isp b and vice versa

just try this:

* setup second dns inside ip range isp b, let root servers know that you have further dns
* give all needed servers second offical ip from isp b
* setup the dns proxy on fw-1 properly to be able to handle both ip addresses, use low ttl

if isp a is down fw-1 will recognise it and will only reponse with ip-adresses from isp b or vice versa