Why does consecutive permutations or consecutive substitutions not enhance security?
The reason two permutations in a row don't increase security is that there is always a third permutation that's equivalent to performing the two permutations in a row. For instance, suppose we have 5 elements, and the following two permutations:
$$\begin{align}
\sigma_1 &amp;: 12345\to34251 \\
\sigma_2 &amp;: 12345\to43512
\end{align}$$

Then if we apply $\sigma_1$ followed by $\sigma_2$, we send $1$ to $5$ by $\sigma_1$ and then to $3$ by $\sigma_2$; if we work out all of the 5 elements, we get
$$\sigma_2\circ\sigma_1 :12345\to52134$$

This is just another permutation (using more common cycle notation, $\sigma_1=(13245)$, and $\sigma_2=(14)(235)$, so $\sigma_2\circ\sigma_1 =(1543)(2)$). As you can see, composing the two permutations just popped out a third permutation; from an attacker's point of view, the fact that we did two permutations is irrelevant, because he can treat it as just one permutation.

With substitutions, it's similar: composing two substitutions gives a third (single) substitution, so there's no difference from an attackers point of view.