Do not insert calls. You also avoid some memory issues when javascript maintains a copy of all local variables for each anonymous function. By making two calls separately, you can also make both calls at the same time, rather than sequentially.
This is the best method if I understand your purpose. Plus, by polling gotA and gotB you can even make a nice little "wait for A notification awaiting B" for users.
: .
var gotA, gotB;
var followingA, followingB;
function getCommonFollowers(user1, user2)
{
gotA = false;
gotB = false;
jQuery.getJSON('http://twitter.com/followers/ids.json?screen_name=/…'+ user1 + '&callback=?', gotFollowersOfA );
jQuery.getJSON('http://twitter.com/followers/ids.json?screen_name=/…'+ user2 + '&callback=?', gotFollowerOfB );
}
function gotFollowersOfA(data)
{
followingA = data;
gotA = true;
if (gotB) {
compareFollowersAB();
}
}
function gotFollowersOfB(data)
{
followingB = data;
gotB = true;
if (gotA) {
compareFollowersAB();
}
}
function compareFollowersAB()
{
f2=followingA;
f1=followingB;
for(var i=0; i < f1.length; i++) {
for(var j=0; j < f2.length; j++) {
if (f1[i] == f2[j]) {
common.push(f1[i]);
}
}
}
$('#content').append(''+common.length+'');
}