[Tex/LaTex] Arrows between arrows

Question

Here is a fragment from MacLane's classic book "Categories for the working mathematician":

Here is another one that's a bit more complicated:

Is there a package that allows to (hopefully easily) typeset things of this kind?

PS: I've been using several packages for making commutative diagrams (amscd, tikz-cd), which is enough for a lot of stuff in category theory. But the picture above isn't really a "commutative diagram", it's something else. I don't even know how to call it, so looking it up on the web is kind of problematic. Thanks.

Answer 1

Here is my version also using tikz-cd:

% arara: pdflatex

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}
\[\begin{tikzcd}
C \ar[shift left=7pt, ""{name=UL, below}]{r}{S} \ar[shift right=7pt, ""name=LL]{r}[swap]{T\vphantom{'}} &
B \ar[shift left=7pt, ""{name=UR, below}]{r}{S'} \ar[shift right=7pt, ""name=LR]{r}[swap]{T'} &
A\\
\ar[from=UL, to=LL, "r", shorten <= -2pt, shorten >= -2pt]
\ar[from=UR, to=LR, "r\smash{'}", shorten <= -2pt, shorten >= -2pt]
\end{tikzcd}\]
\[\begin{tikzcd}
C \ar[shift left=10pt, ""{name=UL, below}]{r} \ar[shift right=10pt, ""name=LL]{r} \ar[""name=ML, ""{name=MLL, below}]{r} &
B \ar[shift left=10pt, ""{name=UR, below}]{r} \ar[shift right=10pt, ""name=LR]{r} \ar[""name=MR, ""{name=MLR, below}]{r} &
A\\
\ar[from=UL, to=ML, "\sigma", shorten <= -3pt, shorten >= -3pt]
\ar[from=UR, to=MR, "\sigma\smash{'}", shorten <= -3pt, shorten >= -3pt]
\ar[from=MLL, to=LL, "\tau", shorten <= -3pt, shorten >= -3pt]
\ar[from=MLR, to=LR, "\tau\smash{'}", shorten <= -3pt, shorten >= -3pt]
\end{tikzcd}\]
\end{document}