Label an edge above and below

Question

I want to be able to label an edge with a part of the label above it and the other part below it like in the following example :

For the moment I have this solution:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}

\begin{tikzpicture}[text=black]
  \node [draw, ] (a) at (0,0) {a};
  \node [draw, below left=4 of a] (b) {b};
  \draw[->] (a) to[bend left] node[midway, sloped, above]{to} node[midway, sloped, below]{b} (b.east) ;
\end{tikzpicture}

\end{document}

Where I create tWo midway nodes, one above the edge, the other one below it but I wanted to know if there is a simpler solution.

---

[EDIT] @AndréC solutions gives the following result on Overleaf:

Answer 1

You do not need two nodes for this, one with align=center is sufficient.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}

\begin{tikzpicture}[text=black]
  \path (0,0) node [draw] (a) {a}
  node[draw, below left=4 of a] (b) {b};
  \draw[->] (a) to[bend left] node[sloped, align=center]{to\\
  b} (b.east) ;
\end{tikzpicture}

\end{document}

Answer 2

Since you have placed the nodes between the coordinates, they are automatically placed in the middle of the path and therefore it is useless to ask for it expressly with midway.

For placement, there is the auto=left option that automatically places to the left of the path and by adding swap, it places it to the right.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}

\begin{tikzpicture}[text=black,auto=left]
  \node [draw, ] (a) at (0,0) {a};
  \node [draw, below left=4 of a] (b) {b};
  \draw[->] (a) to[bend left] node[sloped]{to} node[sloped,swap]{b} (b.east) ;
\end{tikzpicture}

\end{document}