This is the typical case for an installer chainer, particularly since the installers are independent. WiX offers a Bootstrapper project type that creates a bootstrapper/downloader/chainer. It would be ideal for your situation.
You can create such a project using the Bootstrapper project template in Visual Studio or by writing the needed XML files by hand. By default, the bootstrapper creates an uninstall entry for itself to be uninstalled and it will uninstall all of its packages. But, if you prefer, you can make the ExePackage permanent (with respect to the bootstrapper).