Arrow from top arrow to bottom arrow

Question

Consider:

\documentclass[14pt,reqno,a4paper]{amsart}
\usepackage{extsizes}
\usepackage{blindtext}
\textheight 9.3in \textwidth 6.5in
\calclayout
\usepackage{amsmath,amsthm,amsfonts,amssymb}
\usepackage{hyperref}
\usepackage{mathrsfs}
\usepackage[all]{xy}
\usepackage[normalem]{ulem}
\usepackage{tikz-cd}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}

\usepackage{color}
\pagestyle{myheadings}
\begin{document}
\begin{tikzcd}
(\mathcal{C}/U)^{\text{op}} \arrow[rrr, "{\underline{\text{Mor}_{\mathcal{F}}}(x,y)}", bend left=49] \arrow[rrr, "{\underline{\text{Mor}_{\mathcal{G}}}(\Psi(x),\Psi(y))}"', bend right=49] &  &  & (\text{Set})
\end{tikzcd}
\end{document}

The above code results in the following diagram:

I want to add an arrow from the above arrow to the below arrow.

How do I add that arrow? What more can be done in the code that I have written rather than writing totally different code?

Answer 1

Just give a name to the labels and add an arrow.

See Section 2.3 Absolute placement of arrows of the package documentation.

Edit: inserted \newcommand{\uMor}[1]{\operatorname{\underline{Mor_{#1}}}} and shortened Rightarrow, as egreg suggested.

\documentclass[14pt,reqno,a4paper]{amsart}
\usepackage{extsizes}
\usepackage{blindtext}
\textheight 9.3in \textwidth 6.5in
\calclayout
\usepackage{amsmath,amsthm,amsfonts,amssymb}
\usepackage{hyperref}
\usepackage{mathrsfs}
\usepackage[all]{xy}
\usepackage[normalem]{ulem}
\usepackage{tikz-cd}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\newcommand{\uMor}[1]{\operatorname{\underline{Mor_{#1}}}}
\usepackage{color}
\pagestyle{myheadings}
\begin{document}
\begin{tikzcd}
(\mathcal{C}/U)^{\text{op}} \arrow[rrr, "{\uMor{\mathcal{F}}(x,y)}"{name=U}, bend left=49] \arrow[rrr, "{\uMor{\mathcal{G}}(\Psi(x),\Psi(y))}"'{name=D}, bend right=49] &  &  & (\text{Set})
\arrow[Rightarrow, from=U, to=D, shorten >=2pt, shorten <=2pt]
\end{tikzcd}
\end{document}