On pourrait le faire manuellement :
\documentclass[french]{beamer}
%\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
%\usepackage{multicol}
\usepackage{mathtools, stmaryrd, amsmath}
%\usepackage{amssymb}
%\usepackage{xcolor}
\usetheme{Warsaw}
\setbeamertemplate{theorems}[numbered]
\setbeamertemplate{block}[numbered]
\usepackage{lipsum}
\usepackage[french]{babel}
\setbeamertemplate{navigation symbols}{}
%end multi-columns footnote
\begin{document}
\author{Auteur}
\title{Titre}
\institute{}
\date{}
{
\renewcommand{\thefootnote}{\alph{footnote}}
\begin{frame}
\frametitle{Propriétés}
\begin{block}{}
\begin{enumerate}
\item T auto-adjoint \footnote<+->[frame]{$T^* = T$\hfill\visible<2->{\parbox{.45\textwidth}{b. $T^* = T$ et $\langle Tx, x \rangle \geqslant 0, \forall \, x \in H$}}} $\implies \sigma(T) \subset \mathbb{R}$ ;
\item T positif \footnotemark[2]\refstepcounter{footnote}$\iff \sigma(T) \subset \mathbb{R}_+$ ;
\item T unitaire \footnote<3->[frame]{$T^*T=TT^* = Id$\hfill\visible<4->{\parbox{.45\textwidth}{d. $T^*T=TT^*$}}} $\implies \sigma(T) \subset \mathbb{T}$ ;
\item T normal \footnotemark[4]\refstepcounter{footnote} $\implies \lVert T \rVert = r(T)$ ;
\item $\lVert T \rVert^2 = \lVert T^* T \rVert = r(T^* T)$.
\end{enumerate}
\end{block}
\end{frame}
}
\begin{frame}
\lipsum[1]\footnote{\lipsum[2]}
\end{frame}
\end{document}
