While Stardock Central does not allow you to do this on it's own, you can get around it by doing this:
1. Go to settings and uncheck this box:
2. Install ObjectDock 1.90 to a specific folder (after download it will bring up the installer where you can select where to install it).
3. Install ObjectDock 2 to a different folder.
That's the only provided way to do this, and I doubt that will change in the future.