The world of NES collecting is full of unique hardware and software, and it's not unusual to see odd-looking, off-brand cartridges and consoles, most of which were unlicensed clones designed to cash-in on the popularity of Nintendo's 8-bit system.
When famed retro YouTuber John Riggs showed off a two-sided NES cart a while back, it was tempted to assume that this was yet another odd pirate cart from the Far East—but actually, it's a new design by modder @Hey_Its_Thor.
Riggs has been sitting on the cart for a while but has now popped some games inside and applied a label. He's chosen DuckTales and DuckTales 2, a combo which serves as a neat way of quickly accessing two of the NES' best platformers.
We assume Riggs has simply removed the circuit boards from the original carts and installed them in this new double-ended cartridge [Edit: he's confirmed these are repro boards) — if you've ever cracked open a NES cart, you'll know there's a lot of empty space inside those things.
As some people have quite rightly pointed out, Atari had this kind of thing many years ago: