Here is what can be done:
\begin{twocollist}\itemsep=0pt
-\item{{\bf Addition}}{a wxTimeSpan or wxDateSpan can be added to wxDateTime
+\twocolitem{{\bf Addition}}{a wxTimeSpan or wxDateSpan can be added to wxDateTime
resulting in a new wxDateTime object and also 2 objects of the same span class
can be added together giving another object of the smae class.}
-\item{{\bf Substraction}}{the same types of operations as above are
+\twocolitem{{\bf Substraction}}{the same types of operations as above are
allowed and, additionally, a difference between two wxDateTime objects can be
taken and this will yield wxTimeSpan.}
-\item{{\bf Multiplication}}{a wxTimeSpan or wxDateSpan object can be
+\twocolitem{{\bf Multiplication}}{a wxTimeSpan or wxDateSpan object can be
multiplied by an integer number resulting in an object of the same type.}
-\item{{\bf Unary minus}}{a wxTimeSpan or wxDateSpan object may finally be
+\twocolitem{{\bf Unary minus}}{a wxTimeSpan or wxDateSpan object may finally be
negated giving an interval of the same magnitude but of opposite time
direction.}
\end{twocollist}