There are a few choices with Animal Crossing and multiple players. But, to be honest, it's not very well set up (in my opinion) for multiplay.
If you buy a single cartridge, then you'll be able to use it on both devices separately (obviously not at the same time!). Each device will have what basically amounts to a separate game, with it's own island. If you set up a second account on each device for the other daughter, they would be able to use that account to visit the island of the owner of that Switch. These second accounts would also enable them to both play on one device in single screen co-op. But the second player is really limited. All they can really do is pick stuff up and shake trees, and if they leave the screen the main player is on, they will just get teleported back.
If you buy two cartridges, then they will both be able to play it at the same time as each other. They will also be able to visit each others islands in a more independent way - although from what I understand it's still pretty limited. They aren't tethered to the owner any more, but they still can't build anything, it's more of a visit and guided tour deal.
The one bit I don't know is this: To play the multiplayer across two devices, do they need to have the Nintendo online account subscription, even if it's over a LAN? I think maybe they do - but not sure.
EDIT: Just done a quick google, and I think I'm wrong. Nintendo online isn't needed if they're on the same wifi.