Can two \vtop boxes exist side by side?

What you need to know for doing a "box arithmetic" is

• TeX changes vertical to horizontal mode at some places and returns back at others places.

• \vbox or \vtop has its width given as most wider element in it. If this is whole paragraph then the width is \hsize. So it is typical to set a width of \vbox or \vtop by \vtop{\hsize= something ...}

When TeX is in vertical mode, then \vbox{...}\vbox{...} places boxes one above second. But when TeX is in horizontal mode then \vbox{...}\vbox{...} places boxes at the same line. For example:

\leavevmode \vtop{...}\vtop{...} % they are at one line 
                                 % in the paragraph started by \leavevmode 

\hbox{\vtop{...}\vtop{...}}      % they are at one line in the \hbox.

I wrote about TeX modes and boxes in detail in TeXbook naruby, sections 3.4 and 3.5.

IMHO, the "box arithmetic" principles and vertical/horizontal modes are basic knowledges of each TeX users. Unfortunately, LaTeX manuals hides them. LaTeX provides only a complicated construct like \begin{minipage}...\end{minipage}.

You can use standard \parbox and \makebox commands.

There is a subtle point that you seem to be overlooking: the baselineskip might be not uniform with a \vtop (or \parbox[t], which is the same) approach.

\documentclass[fontsize=13pt,letterpaper]{scrartcl}
\usepackage[textwidth=5in,showframe]{geometry}
\usepackage{fontspec}
\setmainfont{Alegreya}[SmallCapsFont=Alegreya SC]

\setlength{\parindent}{0pt}

\newsavebox{\feldA}\newsavebox{\feldB}

\newcommand{\Lcv}[2]{%
  \par
  \hspace*{-1.3in}%
  \makebox[2in][r]{\itshape #1}%
  \hspace*{0.5in}%
  \makebox[3in][l]{#2}%
  \par
}

\newcommand{\Lup}[2]{%
  \sbox\feldA{\parbox[t]{2in}{\itshape#1\par\xdef\tpdA{\the\prevdepth}}}%
  \sbox\feldB{\parbox[t]{3in}{#2\par\xdef\tpdB{\the\prevdepth}}}%
  \ifdim\dp\feldA=\dp\feldB
    \ifdim\tpdA>\tpdB\let\tpdF\tpdA\else\let\tpdF\tpdB\fi
  \else
    \ifdim\dp\feldA>\dp\feldB
      \let\tpdF\tpdA
    \else
      \let\tpdF\tpdB
    \fi
  \fi
  \Lcv{\usebox{\feldA}}{\usebox{\feldB}}\par
  \prevdepth\tpdF
}

\begin{document}

\Lup{Hello this is your Crazy Uncle Unnamed ddd ddd ddd}
    {Who joined the mob to feel good about himself}

Hello and \\ Welcome to {\itshape my} World!!

\Lup{Hello this is your Crazy Uncle Harry yyy yyy yyy yyy}
    {Who joined the mob to feel good about himself}

Hello and \\ Welcome to {\itshape my} World!!

\end{document}

It's easy to see what goes wrong without the fuss with \prevdepth: just comment the line \prevdepth=\tpdF and you'll get