Updated 8/26/19: Updated language to reflect the method below works.
The Nintendo Switch is one of the only gaming systems that you can routinely find more than one of in a household, and you can save some cash by sharing games between the two.
Over the weekend, a Redditor pointed out that game sharing functions on Switch much like it does on Xbox One and PlayStation 4, allowing two systems to play the same game at the same time. To do it, one system needs to be signed into the account used to purchase the digital game. Then log into the eShop and deregister that Switch as the primary system. On a second system, log into the account with the game in question. This will make the second console the account’s primary system. You can then log out of that account on the second Switch, and create a new profile. You can now play the same game on both systems at the same time. If you’re still having trouble, check out our nifty Nintendo Switch game share guide.
There are some caveats to doing this. Local wireless cooperative play does not work, and if you lose your internet connection on the non-primary console, you will also lose access to the game you’re playing until a connection is reestablished. If you play online, you should still be able to compete against each other, provided that the other person is signed into a separate account.
The primary purpose of the game sharing system is for multiple people in the same house to share their game libraries, but not necessarily to play the games at the same time. By not allowing the systems to play cooperatively through local play, Nintendo clearly wants to encourage players to purchase another digital copy of a game. That being said, if you and your housemate want to play the same game but at different times, you can use the game share feature as originally intended and skip the convoluted workaround.
Knowing Nintendo, this could be a feature that gets patched out, but users on the forum ResetEra have found success with it as early as 2018. With the Switch Lite releasing soon and possibly giving households an excuse to buy another Switch, it could allow for more people to enjoy Nintendo games without breaking the bank.