You can not call an MSI from another MSI (from the execute phase specifically). Two MSI's can not run at the same time. The second one will fail.
There is of course nesting which makes the secondary MSI's a child of the main install. However even Microsoft who designed this feature says do not use this. Managing Nested installs is a nightmare.
I do know some people have gotten MSI to work as a bootstrap for multiple MSI's. I am assuming they did all the execution out of the UI phase which begs the question of how they got Maintenance and Silent installs to work. I can not speak to this but there are probably some posts on the forums about it.
What we are looking to do here is update to InstallShield 2012 and evaluating using the new Suite Project type. It seems to be designed for stringing multiple installs together.
