LCDs work on polarization... you typically have an unpolarized source, run it through a polarized filter which is where your 50% loss comes from, then the LCD changes or doesn't the polarization, and you run it through a filter again. If you stacked a second LCD, you shouldn't see a second 50% loss, the light is already polarized by the first LCD.

You are right - the 50% loss comes from the filters, but I forgot to count the loss from the filters. You could add a second monochrome LCD on top of the first one for the same result.

I must be wrong, because this sounds very trivial.