How to draw a 2-cell using xymatrix?

Question

How to do something like this

using xymatrix?

Answer 1

with tikz-cd (its syntax is to my opinion less cryptic as at xymatrix):

\documentclass[margin=3mm]{standalone}
\usepackage{tikz-cd}

\begin{document}
\begin{tikzcd}
A \ar[r, bend  left=60, ""{name=U}]
  \ar[r, bend right=60, ""{name=D,below}]
        & B \ar[Rightarrow, from=U, to=D, "\varphi" description]
\end{tikzcd}
\end{document}

for details see tikz-cd documentation, page 8 and 11.

Answer 2

I adding, now, my proposal with xymatrix package.

\documentclass[a4paper,12pt]{standalone}
\usepackage{amsmath,amssymb}
\usepackage[all]{xy}
\begin{document}
\xymatrix@C=1pc@R=1.5pc{ 
X 
\ar@/^1.5pc/[rr]_{\quad}^{f}="1" 
\ar@/_1.5pc/[rr]_{g}="2" 
&& Y 
\ar@{}"1";"2"|(0.135){\,}="7" 
\ar@{}"1";"2"|(0.875){\,}="8" 
\ar@{=>}|-{\varphi}"7" ;"8"}
\end{document}