Having two or more cards on the same subnet should not cause any problems with duplex/speed settings, but it *will* cause problems with IP routing, unless you use APA and/or ServiceGuard.
(With "ndd -set /dev/ip ip_strong_es_model 1" you can make it work somewhat, but having two NICs simultaneously active on the same network segment without APA is an unsupported configuration and is not recommended.)
With APA you can get both cards active together. ServiceGuard can use APA if both are installed on the same server.
With just ServiceGuard only one of the cards can be active at a time. If it fails, SG will switch the traffic (and the IP addresses) to another card.
Without knowing the hardware type of your NICs and your HP-UX version & Quality Pack level, it's hard to guess what might be wrong.
The output of commands "ioscan -fnkClan" and "lanscan -v" would be very informative. If one of the cards is built-in to the system board of your server, the output of "model" command might be useful too.
MK
MK