If you're switching between base URL's it might just be easier to initialise a new manager each time rather than use a shared one. As much of the benefit of using a shared manager is the single static base URL.
AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
[manager GET:@"http://example.com/resources.json" parameters:nil success:^(AFHTTPRequestOperation *operation, id responseObject) {
NSLog(@"JSON: %@", responseObject);
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(@"Error: %@", error);
}];
The point of dispatch once is to execute the block only a single time.