@aandrew7 capacitive touch (which is the most commonly used touch tech at the moment) is reading changes in the electromagnetic field of the touch matrix caused by say your finger.
This means that ANY material that is conductive will disable the touch functionality.
This is why capacitive touch works through glass, wood and paper etc
Most mirrors (regular and two-ways) rely on some sort of metal coating to get the reflection properties and this is what disable the capacitive touch.
For the diy home user the best way to get touch on a magicmirror is by using IR touch, or something like the airbar, but that has limited functionality at the moment.