Il suffit de redéfinir la commande responsable de la création de la « page » d'abstract, comme dans l'ECM suivant :
\documentclass[mainlanguage=english]{yathesis}
\usepackage{lipsum}
\makeatletter
\renewcommand{\YAD@abstract@page}{%
\bgroup%
\enlargethispage{2cm}%
\setlength{\parindent}{0pt}%
\begin{minipage}{\linewidth}
\small%
{%
\bfseries%
\YAD@ifemptyorundef{\printtitle}{%
\YAD@generic@argument@translate{title}%
}{%
\textsc{\printtitle}%
}%
\ifundef{\printsubtitle}{%
}{%
\newline%
\printsubtitle%
}%
\vspace{-.5em}%
\vspace{\z@}%
\begin{center}%
\YAD@abstractname@mainlanguage%
\end{center}%
}%
\par%
\ifboolexpr{%
bool {YAD@abstract@used}%
and not bool {YAD@main@abstract@empty}%
}{%
\usebox{\YAD@abstract@mainlanguage}%
}{%
\YAD@generic@argument@translate{abstract}%
}%
\ifundef{\printkeywords}{%
}{%
\vspace{.5em}%
\begin{description}
\item[\YAD@translation{keywords}\IfLanguageName{french}{\ifundef{\Fcolonspace}{\FBcolonspace}{\Fcolonspace}}{}:]
\printkeywords
\end{description}
}%
\end{minipage}%
\ifbool{YAD@second@abstract@used}{%
\\[.5\baselineskip]%
\YAD@abstract@page@rule%
\cleardoublepage
\expandafter\selectlanguage\expandafter{\YAD@secondarylanguage}%
\\[.5\baselineskip]%
\YAD@abstract@page@rule%
\cleardoublepage
\begin{minipage}{\linewidth}
\small%
{%
\bfseries%
\YAD@ifemptyorundef{\printtitle}{%
\YAD@generic@argument@translate{title}%
}{%
\textsc{\printtitle}%
}%
\ifundef{\printsubtitle}{%
}{%
\newline%
\printsubtitle%
}%
\vspace{-.5em}%
\vspace{\z@}%
\begin{center}%
\YAD@abstractname@secondarylanguage%
\end{center}%
}%
\par%
\ifboolexpr{%
bool {YAD@abstract@used}%
and not bool {YAD@second@abstract@empty}%
}{%
\usebox{\YAD@abstract@secondarylanguage}%
}{%
\YAD@generic@argument@translate{abstract}%
}%
\ifundef{\printkeywords}{%
}{%
\vspace{.5em}%
\begin{description}
\item[\YAD@translation{keywords}\IfLanguageName{french}{\ifundef{\Fcolonspace}{\FBcolonspace}{\Fcolonspace}}{}:]
\textnormal{\printkeywords}
\end{description}
}%
\end{minipage}%
}{%
}%
\\[.5\baselineskip]%
\YAD@abstract@page@rule%
\egroup%
}%
\makeatother
\keywords{chaos, laugh}{chaos, rire}
\begin{abstract}
\lipsum[1]
\end{abstract}
\begin{abstract}
\lipsum[2]
\end{abstract}
\begin{document}
% \maketitle
\makeabstract
\end{document}
\end{document}
