Unfortunately I don't seem to be able to search for others with this problem because the forum search engine ignores the word 'prerequisite'.
If the prerequisite is not present then the installer needs to run two programs in order: an application installer and its patch.
I tried putting the two 'Products' in the prerequisites page but the first time the installer is run I see a dialog saying there was an error which is why the patch wasn't run. However, if I run the installer a second time it does apply the patch.
Any ideas how to deal with that?