Pc 1 : netbook with an usb wifi antenna spotted on a free wifi hotspot
Pc 2 : same model netbook that can't access internet
What I need : Share the wifi connection by the 2 internal WLAN on ubuntu by network-manager
Looks like you need to configure an ad-hoc wirless network with the two internal wifi cards and route the traffic out through the other wireless card.
You can try this method as described here.
At the terminal install execute sudo apt-get install dnsmasq-base
and then sudo apt-get remove dnsmasq
Restart the NetworkManager by executing sudo /etc/init.d/network-manager restart
Left-click on the NetworkManager icon and click "Create New Wireless Network"
Name it "UbuntuAdhoc" and set the encryption to "WEP40"
Connect PC2 using Ad-Hoc to the PC1