It's not really "clustering" in that they will not automatically distribute user access (you'll need another tool or device to do that, like a network load balancer - see the QlikView Server Reference Manual), but you can install multiple web servers that access a single QVS. Just install them on multiple platforms and then configure each one of them in the QMC. Use QMC->System->Setup->QlikView Web Servers->click the green + icon in the top right corner.
