Two reactions which speed each other up
Have a look at some non-linear reaction schemes such as gas phase radical chain reactions that may lead to explosion. The propagation steps in the hydrogen/bromine gas phase reaction do what you suggest:

$$\begin{align}
\ce{H^. + Br2 &amp;-&gt; HBr + Br^.} \\
\ce{Br^. +H2 &amp;-&gt; HBr + H^.}
\end{align}$$