While I'm using Lichtknarre because I had Wii guns, only a few of us on BYOAC are using it and I don't think anyone has gone all the way through the process of getting two players working with a front-end and various emulators. While I like that it is wireless, that also adds the need to pair over bluetooth before playing.
Yeah, at the moment, any application which
requires mouse input will
not work
w/ 2 players and Lichtknarre. MAME, or anything else which can utilize an analog stick device for absolute cursor positioning should work fine for two players with the vJoy plugin. The other shortcoming is that even for one player mouse input, certain software which requires RAW input will not currently work with it (but I have not found this to be too limiting, given the number of emulator options available.) It's still in beta, and the author of the software is actively looking for solutions to these specific situations. I get the feeling, however, that any solution to this will ultimately require permanently disabling driver signing in the OS, so it may not be a great option in the end for the best 2-player compatibility on a "daily driver" machine.
If overall better compatibility for 2-players is important, you don't care about wireless and the thought of building your own gun doesn't scare you, you probably won't currently find a better solution than Gun4IR. The SAMCO solution may be good as well, as it uses the same underlying technology as the other two, but I get the, perhaps unfounded, feeling that it is currently less extensively developed than Gun4IR.
The short of it is, as BadMouth wrote, thanks to developers of the above, not to mention everyone who has participated in fleshing out the technology before them, there are actually options now which can deliver a near true light-gun experience.
I also don't know an awful lot about the Sinden, but my take on that one is that it's theoretically possible to get closer to the screen, given that the solid screen borders provide an absolute continuous reference. But this relies on overlays or program modifications to get the borders, and they are visible to both the player and the camera, along with all of the other displayed graphics present on the screen. For this reason, I don't feel like it's a better option, either for visual aesthetics during gameplay or for accuracy, given the possible difficulties of accurately separating the border from the rest of the screen. It's also more involved on the software side of things than the other solutions and is pretty pricey. But if you don't want to have IR LEDs attached to your screen, it's the only decent solution which doesn't require them, and it may scale better to really large screens given what's involved in making the super high-output IR beacons required for the greater distances necessary.
*edit* Here's a video showing some line of sight gameplay with the Sinden. Seems like it does pretty well on this individuals setup...