One NLB should be enough for this.
Let say you have two WFEs, WFE1 and WFE2 and two web applications webapplication1 and webapplication2.
Lets work with first Web Application webapplication1.
You have configured AAM for webapplication1 with an internet name http://internetWebApp1 for internet users and Extranet name http://extranetWebApp1 for extranet users. Both URLs are different.
Now put an entery in NLB to point 50% of the traffic for the requests of http://internetWebApp1 on WFE1 and 50% on the WFE2.
Put another entery in NLB to point 50% of the traffic for the requests of http://extranetWebApp1 on WFE1 and 50% on the WFE2.
Same process can be followed for webapplication2.
SharePoint will automatically assist the requests for both URL's. NLB has only to tell what is the SharePoint server name/IP against that URL
Hope it helps.