Align left and right same line

A \hfill that occurs at the start of a line will be ignored. To avoid this, use \null\hfill in place of \hfill.

Don't use \hfill for the alignment, use \flushleft and \flushright:

\begin{minipage}[t]{7cm}
\flushleft
\textsc{Relator}

Prof. NameName\\
\end{minipage}
\hfill
\begin{minipage}[t]{7cm}
\flushright
\textsc{Candidato}

Name Name\\
Matricola: XXXXX
\end{minipage}