Well Facebook would prefer you have a button somewhere that users have to tap to fire off that 2nd login. User engagement is higher when the are not overwhelmed at the beginning of your app launch requesting permissions. They would prefer the extra permissions be delayed until you actually need them, but you could call login again from the call back function. However you have to be careful with how you programming that call back function because the second login calls the same call back function (though technically you should be able to have a different function for the 2nd login…).
Rob