Concurrent / Nested Installations: I don't have Installshield available right now, so this will be "best effort" - seeing as nobody else has answered. You should not kick off other MSI files from within an MSI file. There are very concrete technical reasons for why this is always wrong. Essentially two MSI installation transaction sequences are not allowed at the same time - a runtime error results. Some details: Concurrent MSI installations. Instead you need to launch each MSI in sequence from some launch mechanism.
Installshield Suite Projects: Please research Installshield's Suite Projects. I think you need the Premier edition for this feature to be available. Please check the screenshot in the provided link (towards bottom). These suite projects will allow you to run different MSI files and EXE files and a number of other file types "in sequence" in a more reliable fashion.
Shared Installation Folder: I do not have Installshield Suite Projects available to try right now, so I can't really answer your question on how to enforce a shared installation folder. Frankly it is a little bit unclear what you mean with that - seeing as each setup would generally need its own, unique installation folder? Still, I believe the Suite Projects will provide its own GUI, potentially suppressing the individual MSI setup's own GUI and passing in silent installation parameter for the MSI's command line. You should be able to control these installation parameters programmatically from within the Suite Project and hence be able to enforce a common destination folder.
