Define command that accepts \\ in arguments

Question

I am trying to define a new command \dfix that can handle newlines in its arguments. The command works as expected when the arguments have no \\ in them, but LaTeX produces errors if I do include newline characters.

How can I create a command/macro that is safe with regard to line breaks?

I would like \dfix in the following context

\begin{align*}
  &\dfix{(A\\
  &B)}{C}
\end{align*

to produce the same output as

\begin{align*}
  &(A\\
  &B)^\ddag_{C}
\end{align*}

Similarly, $\dfix{A}{B}$ should produce the same output as $A^\ddag_B$.

Here is a MWE defining the command and trying to us it in various places.

\documentclass{minimal}
\usepackage{amsmath}
\newcommand{\dfix}[2]{\ensuremath{{#1}^\ddag_{#2}}}
\begin{document}
% This causes: ./mwe.tex:9: Missing } inserted.
\begin{align*}
  &\dfix{A\\
  &B}{C}
\end{align*}
% This is OK
\begin{align*}
  &\dfix{A B}{C}
\end{align*}
% This also produces errors.
\begin{multline*}
  \dfix{A\\
    B}{C}
\end{multline*}
\end{document}

Answer 1

This does what you ask but seems a very weird construct, are you sure this is what you need?

\documentclass{minimal}
\usepackage{amsmath}
\newcommand{\dfix}[2]{(#1)^\ddag_{#2}}
\begin{document}
% This causes: ./mwe.tex:9: Missing } inserted.
\begin{align*}
  &\dfix{A\\
  &B}{C}
\end{align*}
% This is OK
\begin{align*}
  &\dfix{A B}{C}
\end{align*}
% This also produces errors.
\begin{multline*}
  \dfix{A\\
    B}{C}
\end{multline*}
\end{document}