Support handheld console linking
Some games on Nintendo handhelds, such as Game Boy, support linking two devices together via a cable. Games make use of that to provide two-player modes or item trading.
Gambatte supports this via a network connection: there are options to set a running instance to act as a network server or client. We could run two instances, set one as a server, and connect the other one to it, then forward different controllers to the two instances.