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.