Looks like the prototype works great in the video. Kickstarter in video description- £80 (~$105) minimum to receive product. According to the creator the kickstarter is only to get numbers so he can place required orders for parts. He's already just about at the goal too after only a day.
One drawback is that it requires a white border around the screen so it can see the borders. This border is drawn by the included software (Windows/Linux/R-Pi).
He does say two guns can be used at once. From the Kickstarter:
Quote: The prototype shown is a high quality SLS 3D printed custom design. It features a high speed camera, a CPU, USB interface, a trigger, 4 assignable action buttons and a pump action reload feature. So far it has driver software for Windows and Linux (including Raspberry Pi), it is mapped to behave like a physical mouse for easy software compatability. Providing the software is compatible (such as Mame) you can even use 2 Sinden Lightguns at the same time. The final production version includes an additional 4 assignable buttons (therefore 8 in total) that you can use for navigating menus or assigning additional actions.