Large simulations require the combination of many different applications. In many cases they are described as scientific workflows. In the recent years significant knowledge accumulated in the form of scientific workflows. This helps scientists to build even more complex simulations particularly, if they combine existing simulations with new ones and create multi-workflow simulations. The SHIWA project created the SHIWA Simulation Platform to help scientist sharing and combining workflows to build such complex multi-workflow simulations that also require the use of as many computing resources as possible. The mixed use of grid and cloud infrastructure can provide the necessary amount of resources that are needed to complete complex multi-workflow simulations in reasonable time. The SCI-BUS project has created the gateway technology that enables the mixed use of grid and cloud infrastructures. The current paper describes the SHIWA and SCI-BUS technology and how they can be used to run multi-workflow simulations on the mixed grid and cloud infrastructures.