Combining two signals.

gistfile1.m

[[RACSignal
   combineLatest:@[self.firstNameField.rac_textSignal, self.lastNameField.rac_textSignal]
          reduce:^(NSString *firstName, NSString *lastName){
              return @(firstName.length > 0 && lastName.length > 0);
          }] toProperty:@"enabled" onObject:self.button];