\documentclass[tikz,border=3mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\usepgfplotslibrary{autonode}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
  width=8cm,height=5cm,xmin=0,xmax=1,ymin=0,ymax=1,
  auto node placement,auto node debug=summary]
  \addplot[blue,domain=0:1,samples=21] {0.3}
    \pgfplotsautonode[]{first axis};
\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
\begin{axis}[
  width=8cm,height=5cm,xmin=0,xmax=1,ymin=0,ymax=1,
  auto node placement,auto node debug=summary]
  \addplot[red,domain=0:1,samples=21] {0.7}
    \pgfplotsautonode[]{second axis};
\end{axis}
\end{tikzpicture}
\end{document}