Need uninstall of one package to first uninstall another (nested uninstall?)

[Bob K]

I have a situation where, after installing MAIN.MSI, a user may install
SUBPROD.MSI (for example). SUBPROD.MSI will not install unless MAIN.MSI is
already installed. The user should uninstall SUBPROD.MSI before uninstalling
MAIN.MSI, but I can't guarantee that will happen, so I want to have the
uninstall action of MAIN.MSI FIRST uninstall SUBPROD.MSI.

How do I do that?

To be more accurate, there can actually be any number of additional packages
that have been installed. I won't know what is installed and their product
codes until the uninstall of MAIN.MSI is started.

I thought that using a CA type 39 was the trick. I needed to dynamically
create it and put it in the Execute sequence. Unfortunately, it fails
because the package that it is uninstalling was not installed as a nested
install:

DEBUG: Error 2335: Path: C:\product\Product Configuration 1\CD
Release\DiskImages\DISK1\ is not a parent of C:\WINDOWS\Installer\

Should (or could) I just launch a msiexec /x GUID from the UI sequence? That
is obviously not great for several reasons.

Any ideas would be appreciated.

Thanks,
Bob