Several months ago, I looked at a common hardware component used by bitcoin miners and the thought occurred to me: this might be useful for creating additional seats. So I went to eBay and purchased one, tested it, and uploaded a short video:
The long story short is, it works fantastic. I have a motherboard with only two PCI-Express 16x graphics slots on it, so that's my theoretical limit for seats: 2. But because I also have a single 1x PCI-E slot I can take advantage of that and use the riser to plug in an additional third 16x card, giving me a third seat.
My thoughts are that if at all, this setup would primarily be useful for anybody who has a motherboard with only one single 16x slot on it. That would negate buying a whole new computer or even another motherboard if you are against such a thing. Just add in another 16x slot using a 1x slot. Just be warned, this setup cannot be used for high end gaming on the second slot. One single PCI-Express lane has more than enough bandwidth for day to day desktop or internet usage, but it would be bandwidth starved with a large real-time 3d workload. All gaming would have to be in connection with whatever video card is in the first slot.
EasySeats is open source software developed to bring ease to users who choose to go multi-seat. It supports video cards for seat creation as well as external USB docks.