Total current in two circuits - two methods
In the first case, since \$I_N\$ is obtained by short-cicuiting the terminals, you shirt-circuit all resistors, so you effectively only have the two current sources. In the other example, if you short-circuit the terminals all resistors remain 'active' (in the sense that current flows through them). So your first example is a very special case because it already is almost a Norton equivalent cicuit.