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\setlength\subproofhorizspace{1.3em}
\begin{logicproof}{2}
\exists x \; P(x) \lor \exists x \; Q(x) & \prem\\
\begin{subproof}
\exists x \; P(x) & $\assum$\\
\begin{subproof}
P(x_0) & $\assum$ $\freshVar{$x_0$}$\\
P(x_0) \lor Q(x_0) & $\ori3$\\
\exists x \; (P(x) \lor Q(x)) & $\existi4$
\end{subproof}
\exists x \; (P(x) \lor Q(x)) & $\existe2,3-5$
\end{subproof}
\begin{subproof}
\exists x \; Q(x) & $\assum$\\
\begin{subproof}
Q(x_0) & $\assum$ $\freshVar{$x_0$}$\\
P(x_0) \lor Q(x_0) & $\ori8$\\
\exists x \; (P(x) \lor Q(x)) & $\existi9$
\end{subproof}
\exists x \; (P(x)\lor Q(x)) & $\existe7,8-10$
\end{subproof}
\exists x \; (P(x)\lor Q(x)) & $\ore1,2-6,7-11$
\end{logicproof}