Application deployment problem

I have two Office VSTO customizations I am trying to deploy.

Both have different Solution ID's both have different assembly names and version numbers.

They both install and run without pproblem individually, however if one is already installed the second will not install because it says an applicatiojn already exists with the same name.

The strange thing is if I then uninstall the first one and then re-install them both this is fine. I cannot think what name it thinks already exists and why it would cause this problem.