Multiple ISP connection

[choy <choylists@gmail.com>]

Hi,

I have a web server connected indirectly to the Internet(ISP -> router
-> server eth0). I've setup NAT port forward in the router so all web
server connections are forwarded to server. Recently, I've added a new
ethernet card(eth1) to the server, and it has a public ip(ISP ->
server eth1). After ifup'ing eth1, I can't connect to my server from
old IP(eth0) anymore, but from new ip is OK. If I ifdown eth1, eth0
work as before.

So my question is: how can I config the server so both connection
(eth0 and eth1) can connect to my server?

PS. I've searched google and find something like "Dead Gateway
Detection" but seem not useful for me....

Thank you!