Re: game sharing, the only time you need 2 copies of the game is if your dds will want to play the game together at the same time.
You can share games, both physical card copies or downloaded, but only one switch can be playing the game at any time. For lots of games, this will not be a major problem - eg, with Mario kart, then they can play multiplayer using one switch and one copy of the game.
The only time it is an issue is when both dds want to be playing the same game on different consoles. In Animal crossing, they could have one game card and share it - each creating their own profiles and islands on their own console, or they could each have their own profile on one console and share an island. The snag comes if they want to visit each other’s islands - you will need 2 copies of the game, as they will both have to be playing at the same time to do this. If they would be happy each having their own island and playing separately, or both sharing one island using 2 profiles on one console, then you will be fine with just one copy.
FWIW, we have a switch (shared family console) and a switch lite (my personal device), and animal crossing is the only game we have 2 copies of. Everything else we share in the usual way (both cards and downloads) as it’s easy enough to not be playing the same game at the same time, and if wanting to play multiplayer (in Mario kart, or Mario party, or the Lego games) it’s easier to play it on one console as multiplayer than across the two consoles linked anyway.