How to link different trees with TikZ

No idea what is broken, but is this workaround OK?

Essentially, use one forest for both trees, with the root as a phantom node. The s sep determines the distance between the two trees.

I don't know if/how nodes are named in forest, so I gave the e and h nodes an alias that I used in the \draw at the end.

\documentclass[12pt]{article}

\usepackage{tikz}
\usetikzlibrary{fit}
\usepackage{forest}

\begin{document}
\begin{forest}
[,phantom,s sep=3cm
[a
  [b
    [c]
    [d]
  ]
   [e,draw,dashed,alias=e]
]
[f
  [g]
  [,phantom]
  [h,alias=h,tikz={\node [draw,dashed,fit=() (!1) (!l)] {};}
    [i]
    [j]
  ]
]
]
\draw [-stealth](e) to[bend right] (h.west);
\end{forest}
\end{document}

You can use \subnode:

\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{forest}
\usetikzlibrary{tikzmark}
\begin{document}
\begin{minipage}{0.3\textwidth}
\begin{forest}
[a
  [b
    [c,name=c]
    [d,name=d]
  ]
   [\subnode{marke}{e},draw,dashed]
]
\end{forest}
\end{minipage}%
\begin{minipage}{0.3\textwidth}
\begin{forest}
[f
  [g]
  [,phantom]
  [\subnode{markh}{h},name=h,tikz={\node [draw,dashed,fit=() (!1) (!l)] {};}
    [i]
    [j]
  ]
]
\end{forest}
\end{minipage}
\begin{tikzpicture}[overlay,remember picture]
\draw[green](marke.east)--(markh.west);
\end{tikzpicture}
\end{document}