On a DC you should only have one card enabled because of DNS and AD replication issues. There are some configurations you can do to get around this but the recommedation is to still disable one NIC.
Also, I recommend keeping your network infrastructure services like DNS and AD seperate from other services like App server, SQL and IIS. If you can I would keep those seperate. I know some sites are too small but there is always virtual machines that can be installed.
On your secondary NIC you want to go into the TCP\IP settings click advance then DNS tab uncheck register this interface with DNS.
Then follow instructions below.
http://technet.microsoft.com/en-us/library/cc740071(WS.10).aspx