The application ID is what distinguishes apps on your device and in Google Play. I don't think it is possible to force only one app to be installed at a time if they have different application ID. Android has an "application ID" which is used to uniquely identify your app. It has nothing to do with what keystore is used to sign the APK.
But, the application ID is not necessarily the package name. An excerpt from the docs:
Although your project's package name matches the application ID by default, you can change it. However, if you want to change your package name, be aware that the package name (as defined by your project directory structure) should always match the package attribute in the AndroidManifest.xml file
So you can have two different apps with the same application ID, but different package names. And then only one of them could be installed on a device. I'm not sure if that's what you're going for, though.
And I'm not sure that your reasoning for doing this is great. Maybe you can expand upon what you mean by
This is to allow Just one of the two Apps to be installable in a supporting device.
so that maybe someone can suggest a better alternative for your situation.