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Both cards have 3 settings that are of primary importance:
IP Address
Netmask
Gateway (or default route)
Your problem is that both cards have their own GATEWAY. When you plug in the 2nd card, it's changing your routing table and using the 2nd card's default gateway, causing traffic destined for the Internet to traverse the wrong card.
Go into the settings for the 2nd card and delete the Gateway for the 2nd card, only the 1st card should have one.
If the devices connecting to the 2nd card need to access the Internet, make sure that packet forwarding is ON.
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