Den of Snakes relies on the synchronization behavior provided by the DMG-07 4 Player adapter device. The device collects the synchronous link input from up to 4 Game Boys and then packages and re-broadcasts that data to all connected consoles simultaneously.
So if you want it to work online then your device or service would have to emulate the specific behavior of the DMG-07. The documentation about the device is available here: https://gbdev.io/pandocs/Four_Player_Adapter.html
Den of Snakes does not work with two Game Boys and a single link cable. (it is open source and you are welcome to modify it to work differently if you’d like.)
However, if you want to find some other Game Boy homebrew that uses the link cable there are several in the GBDK gallery (link filter turned on): https://gbdk-2020.github.io/gbdk-2020-gallery/?linkOrMultiPlayer=true
For example my game Petris does work two Game Boys and a link cable. Perhaps it already works with what you are working on. (It is also open source and can be modified) https://bbbbbr.itch.io/petris