Authors with multiple affiliations

Here is an option with package authblk

% !TEX encoding = UTF-8 Unicode
% !TEX TS-program = xelatex
\documentclass{article}
\usepackage{authblk}

\begin{document}

\title{title}
\date{}

\author[1, 2]{\small Erwin T. Lau}
\author[3, 5]{\small Massimo Gaspari}
\author[1, 2, 4]{\small Daisuke Nagai}
\author[1, 2, 4]{\small Paolo Coppi}

\affil[1]{\footnotesize Department of Physics, Yale University, New Haven, CT 06520, USA}
\affil[2]{\footnotesize Yale Center for Astronomy and Astrophysics, Yale University, New Haven, CT 06520, USA}
\affil[3]{\footnotesize Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544-1001 USA}
\affil[4]{\footnotesize Department of Astronomy, Yale University, New Haven, CT 06520, USA}
\affil[5]{\footnotesize Einstein and Spitzer Fellow}

\maketitle

\end{document}

you would get

names of authors come from arXiv.

I did a basic redefinition of \author to include an optional argument; the value used in this argument will be appended, together with a comma, to the superscript number used in the affiliation:

\documentclass[nouppercase]{ifmbe}

\makeatletter
\renewcommand{\author}[3][]{
      \stepcounter{ifmbe@authors}
      \expandafter\def\csname ifmbe@author\alph{ifmbe@authors}\endcsname
      {#2$^{\expandafter\the\csname ifmbe@affiliationcounter#3\endcsname
        \if\relax\detokenize{#1}\relax\else,#1\fi}$}
}
\makeatother

\title{Authors With Multiple Affiliations}
\affiliation{First Institution/Department, Affiliation, City, Country }{FIRSTAFF}
\affiliation{Second Institution/Department, Affiliation, City, Country }{SECONDAFF}
\author{A.B. Firstauthor}{FIRSTAFF}
\author{C. Coauthor}{SECONDAFF}
\author[2]{D.E. Othercoauthor}{FIRSTAFF}

\begin{document}

\maketitle

\end{document}

A more general redefinition could be made, but for a one-case situation, this should be enough.