\begin{sequencediagram}
  \newthread[blue]{s1}{:Server1}
  \newinst{db}{:Database}
  \newthread[red]{s2}{:Server2}

  \begin{call}{s1}{reading}{db}{data}
    \postlevel
  \end{call}

  \prelevel\prelevel

  \setthreadbias{east}

  \begin{call}{s2}{reading}{db}{data}
    \postlevel
  \end{call}
\end{sequencediagram}