Recently several distributed rendering systems have been developed which exploit a cluster of commodity computers by connecting host graphics cards over a fast network to form a compositing pipeline. This paper introduces a new algorithm which takes advantage of the programmable compositing operators in these systems to improve the performance of rendering multiple shadow-maps, for example to produce approximate soft shadows. With an nVidia GeForce4 Ti graphics card the new algorithm reduces the number of required render nodes by nearly a factor of four compared with a naive approach. We show results that yield interactive-speed rendering of 32 shadows on a 9-node Sepia2a distributed rendering cluster.