Hey Paul, I'd love to clear up all your concerns. In a nutshell, you most certainly can install an SSD as a second drive in your Mid 2011 Mac Mini. [http://www.ifixit.com/Guide/Installing-Mac-Mini-Mid-2011-Dual-Hard-Drive-Kit/6275/1|The guide] for your Mini actually is showing how to install a second hard drive. In the beginning of the guide, you are removing the first hard drive to get to the spot where you can install the second one. As you progress through the guide, we show you how to install the second hard drive. As with all of our guides, you must then follow it in reverse to reassemble the device. During this reassembly, you will reinstall the first hard drive on top of the second one.
As for Apple not selling a dual hard drive edition of this size Mini, your assumptions are absolutely correct. However, we installed a second one on the exact same model that you have and it worked beautifully.
Hope this helps!
