Dual Ethernet connection

I am Insturmentation guy & working on the PLC.
I am using WinNT with with one SCADA package from the Intellution.
I want to install dual NIC & establish a dual Ethenet communication with the Allen Bradley PLC (SLC5/05)
Now I want to know to have this dual ethernet communication, do I install two NIC with different IP addresses? Whether the two NIC should be of different make & model?
Alreay I tried to install two NIC, both of 3COM make 3C509B model.
But when I installed 2nd one, first onewhich is in 3rd ISA slot got disabled & 2nd one installed in 1st ISA slot is getting recognized for the same IP address of old.
So please advice me to install two NIC & make dual Ethernet connection possible.

Thanks

Rahul.