By the way, previous discussions wondering about double-ended NES/Famicom carts happened in threads below. Both times user Fisher mentioned a brand called Hydron that actually made carts like this. The 2019 post has a link to a blog post (in Portuguese). The 2018 post includes an attached picture (uploaded/hosted on the forum) of an open cartridge showing what the board looks like.
2019: viewtopic.php?p=236004#p236004
2018: viewtopic.php?p=221570#p221570
Video Game Consoles with games stored in single-sided cards
Re: Video Game Consoles with games stored in single-sided cards
It's the case that'd be expensive in that case. Molds are >10k. Then you have to consider how users will take it, collectors will balk at something that looks like obvious, visible cost cutting.