And -- that's why it was "tentatively disagree". MLXXXp, those are invaluable details for sorting this out.
That BeyondLogic reference for USB is fantastic. It reminds me of just how much "not well understood" technology is buried in our current devices. For me, in spite of what I did know about USB, I didn't know it down to this level. Reading stuff like that just sucks me right in for hours at a time.
So - we are not going to be sending any almost-USB packets between a pair of Arduboys connected by a cable.
But hardware can still have hidden talents. Go take a look at the forums here where they are doing multilevel gray scale demos on an Arduboy "black and white" display.
Going back over the datasheet with the BeyondLogic details in my mind, I would test a different approach. It appears that creative use of the DETACH and LSM control bits may be able to cause the USB Data lines to change voltage. On the other end of the cable, this may be sufficient to trigger a WAKEUPI interrupt. If that actually happens, add some creative use of software timers, and you've got the weirdest serial port going. As a technology, this is roughly equivalent to Morse code - but if it works, it may be enough to allow the transmission of a few tens or hundreds of bits between two stock Arduboys connected with a stock OTG Micro/Micro cable. This potential ability would need to be validated in testing - this is way out of plan for the chip and it's architecture, and success is definitely not guaranteed.
I will note that going with a hub PC type of device will absolutely work, and can even have a good shot at enabling not only two-player, but also multi-player, modes, along with easy configuration updates such as levels, and maybe even a tiny portable loader for changing games on the road. With careful thought about what is needed, it could be portable and compact - essentially a Arduboy Pocket Server. At the end of the day, though, it's still another device, and it won't be something users can easily find in any technology store nor are they likely to have a portable version on hand. I would be concerned that although it would be capable, it would be too rare.
Getting around that for some of the users is why I would also want to work out a standard cable solution. If you and a friend discover you both have an Arduboy, the addition of a cable you might find in the cell phone shop is enough to get you into head-to-head mode. Alternatively, a pair of the usual USB micro cables could be back-to-backed with a USB type A gender changer, something that has already been available for years, and something small and inexpensive enough that a dedicated Arduboy gamer might buy and carry one just to have it on hand.
Also - I want to thank you. Involvement like yours is what will make a project like this get up and really run. It's because of responses like yours that I have committed to a buddy pack - can't test those multi-player modes without having several on hand!!!