This is a question that has been floating around for a long while. A while back, Nintendo revealed that Switch owners would be able to share downloaded software between Switch consoles. However, it was never specified how it could be done. Well Nintendo UK have shared the information on their website with a step by step process.
To begin with, the first Nintendo Switch console that you connect to you Nintendo Account will be your primary console. Any Switch consoles that you register to your Nintendo Account afterwards will connect to your your account but will be secondary consoles.
The primary Switch can download and play any software you have purchased on the primary console without the need of an internet connection except for when playing online or using other features that require an online connection.
The secondary Switch can download software already downloaded to the other system that is registered to your Nintendo Account. However, to play this content, you will need a constant internet connection in order to play downloaded content.
This might be so that save data can be uploaded to the cloud and shared between both devices. You also can not play the same downloaded software on both/all consoles at the same time that are linked to the same account. You can use another account on the primary Switch to both play the same game at the same time (thanks to KalaMar for the tip).
We have the full instructions and definitions as shown on the Nintendo UK website below:
What Are the Differences Between a Primary and Non-Primary Nintendo Switch Console?
The first time you use your Nintendo Account to connect to Nintendo eShop for Nintendo Switch, the Nintendo Switch console you use will be registered as the primary console for your Nintendo Account. After you have designated the primary console for your account, you can still link that account to other Nintendo Switch consoles, but these consoles will not be registered as primary consoles.
When using the primary console, any user on that device can play downloadable content you have purchased. You are also not required to connect to the internet prior to using downloadable content on your primary console (an internet connection is required to use online features or games which require a subscription service).
If you deregister your primary console, the next time you use your Nintendo Account to connect to Nintendo eShop on a Nintendo Switch console, that console will become the primary console for your Nintendo Account.
You can have one primary console at a time for your Nintendo Account.
The first Nintendo Switch console you use to connect to Nintendo eShop on Nintendo Switch will become your primary console.
While using the primary console, any user account on the console can play the downloadable content you have purchased.
While using the primary console, you do not need to have an active internet connection while playing downloadable software.
An internet connection is still required to use online features or games which require a subscription service.
You can change which Nintendo Switch console is your primary console, but you must first deregister the current primary console.
A Nintendo Account can be linked to Nintendo Switch consoles that are not designated as the primary console, however, some functionality differs from when a Nintendo Account is used on a primary console.
While using a non-primary console, you can still use the Nintendo Account to access Nintendo eShop, purchase and download software, and in under certain conditions, play your downloadable software.
While using a non-primary console, you must have an active internet connection to play downloadable content.
If you lose your internet connection while playing downloadable content on a non-primary console, your game will pause after a certain amount of time; however, once you connect to the internet again, you will be able to resume from the point you left off.
While using a non-primary console, downloadable content can only be started by the user that purchased the content.
When using downloadable software on a non-primary console, your game will pause if your Nintendo Account is used to access downloadable software on any other Nintendo Switch console.
How It Works:
Registering your primary console:
Link a Nintendo Account to your Nintendo Switch console.
Access Nintendo eShop and select your Nintendo Account.
If the Nintendo Account does not already have a primary console, this console will become the primary console for your Nintendo Account.
Making a different console the primary console:
Access Nintendo eShop on your primary console and deregister the console for your Nintendo Account from your Account Information menu.
If you do not have access to the original console, you can also deregister a primary console off-device once per year.
Off-device deregistration can be done by visiting accounts.nintendo.com, selecting “Shop Menu”, and then “Deregister Primary Console”.
On the second Nintendo Switch console, use the same Nintendo Account to access Nintendo eShop. This is now the registered console for your account.