Hello David,
If you are trying to install side by side different versions of your product, each product version having multiple instances installed, then once again I am afraid this scenario is not supported by our "Multiple Instances" feature. The side by side installation of different product versions requires the "Allow side by side installs of different product versions" option from "Upgrades" page which is not supported by our "Multiple Instances" feature.
In this case you have to either disable the "Multiple Instances" feature or to use the "Automatically upgrade older product versions" option from "Upgrades" page.
In what regards the later related behavior (installer hanging) please make sure that you did generate different ProductCodes for the new version of your setup project. Besides enabling the "Build major upgrades for multiple instances" option. If the behavior still persist, then please forward us by email to support at advancedinstaller dot com the old and new version of your AIP (setup project) so we can check their settings and test the scenario.
All the best,
Daniel