There are several challenges that need to be overcome when trying to attach 2 arcade controllers while at the same time providing for up to 5 external controller ports to be used simultaneously. This is greater complicated by allowing all of the controllers to communicate with one of the four active SNES motherboards at any given time.
I'm currently working on the third revision of the control switcher circuit. Last night I built the prototype.
The circuit is relatively simple and allows me to enable or disable controls for the active game as well as allowing the software to toggle the reset line. This circuit is controlled by using two IO lines on the Arduino.
One benefit of this design allows the system to optionally enable or disable control based upon whether a coin has been inserted. It's not likely that I'll ever operate outside of free play mode, but I suppose this feature was added to satisfy my own curiosity more than out of any real utility.
Allows coin-up support.
Allows controls to be isolated from any inactive SNES console.
Allows reset to be toggled programmatically.
Next up: Handling of video switching/(potentially) using one RGB amp for all consoles.