The Nintendo Vs. Dualsystem was an early arcade platform that had the unique ability of running two separate copies of a single program on one boardset.
If you don't know what I am talking about then let me give you an example. If you have Quake installed on your computer, you can open up multiple copies of it at the same time, and run them in different windows. This is what the Vs. Dualsystem did. It would run two games concurrently, and feed them out to separate monitors and control panels. A Dualsystem with a single game installed will feed that game to both of its monitors. But you can install a second game on the board, and then you can run either game on either monitor (selectable via dip switch).
The Dualsystem is similar in design to the Nintendo Vs. Unisystem, but is a separate platform, and required different software than the Unisystem did, but this is a moot point, because many Unisystem games were ported to the Dualsystem as well. The Dualsystem hardware was rather esoteric, and is not nearly as well documented as the Unisystem is.
Some Dualsystem games utilized the hardware better than others. Tennis for example allowed for linked play between the two game monitors. While other titles like Golf and Mach Rider and Excitebike did not. Oddly enough the Tennis game required two sets of Tennis chips for linked play, but could run two unlinked copies from a single set of EPROMs (curiouser and curiouser).
The Vs. Dualsystem was available in both upright and cocktail format. The uprights were grey or light blue, and looked like two arcade cabinets that were joined at a slight angle. The cocktails had two monitors that were housed in an enclosure, with each player getting her own monitor (these are not a true cocktail, because there is no actual "table" part, as the monitor enclosure takes that all up.
Both of these cabinets are very rare today. The uprights were often refitted with two complete wiring harnesses to run two newer games. A few years back I encountered one of these that was running Streetfighter II on each side (doing a conversion like that requires two of everything, while the original Dualsystem only had a single PCB, power supply, etc). Over time most of the uprights have been junked, as oversized cabinets are usually easily damaged, and very hard to sell used (they weigh about 400lbs, and are nearly impossible to manuever in a normal house). It has been at least three years since I have seen one of these at all (in any form), and it has been seven years since I saw one that was still running the original games.
The cocktails are even rarer than the uprights. I have never encountered one of these out in the real world at all. I did nearly purchase on on eBay a while back (it was local, and the seller refused to ship due to weight). I thought I had it locked in as I had been the high bidder for days, but 4 new bidders entered in the last minute, and I ended up losing it (high bid was $202.51, my max bid was $200.01).
The actual list of Dualsystem titles is still disputed. I have seen Super Mario Bros., Golf, Tennis, Ice Climber, Duck Hunt, Mach Rider, and Excitebike in real life. There were probably a lot more than that, but those are the only ones I am sure about. Most of the Dualsystem games are unemulated or incorrectly emulated, so don't bother trying to use MAME to play these. But luckily all known Dualsystem titles were also available on the Nintendo Vs. Unisystem (which is emulated).