As soon as I inserted my USB card it was simple plug and play. So, when I connected to my AP both card (internal and USB) would connect to it. My access point will show two devices connected to it (both the names point to my laptop). So when I tried to disable my internal card, I used the following steps:
Add the following line to /etc/network/interfaces:
iface wlan0 inet manual
We can replace wlan0 with the interface name that we want to disable. It worked there was no sign of my internal card. So when I again pluged my USB card the simple plug and play method won't work anymore. But when I enabled my internal USB card it started working again but same as earlier both card would connect to my AP.
How can I disable my internal card and make use of my new USB card.