Now, I believe the MSI engine itself enforces installing, modifying, or removing only one program at a time - if you try to run an .msi while another is uninstalling, for example, it either won't run or will wait for the currently running uninstall to finish. Non-MSI installers may not behave this way - since they don't use the MSI engine. But because of this safety design decision, this is probably why appwiz.cpl insists on only letting one uninstaller be called at once.