Advanced Installer should monitor the .aip file it has open for changes in the background. If changes are detected, it should prompt to reload the file. There are two different prompts that can be shown: One when there are no unsaved changes, and a more severe prompt when there are unsaved changes (eg: either reload from disk and lose your changes, or ignore the changes from the disk version and keep your changes). VisualStudio does this, for example.
May not be that common for two devs to be working on installers, but I can see it happening in a large-ish product made of many components (like mine).
Scenario:
Two developers using subversion and tortoisesvn.
- Developer A opens .aip file, and makes some changes
- Developer B opens .aip file, and makes some changes to a different part, and saves it.
- Developer B commits to SVN.
- Developer A is working on several other pieces of product, and as a matter of consequence does an svn update - which gets them the .aip file with Developer B's changes.
- Developer A finishes working on .aip, and saves it.