This is something I have been wondering about recently. Will the Switch have single card download play like the DS and 3DS? Or will games have a free multiplayer download version on the eShop? I liked that solution on new Mario Party for 3DS.
Download play was one of the best features of DS and 3DS, it would be annoying if every Switch game was multi card play only.
GameSpot asked Reggie about it in an interview last month. First he says that it would come down to the developer and the type of content they want to create (which makes it sound like it
is possible), but then he ends by saying "the ability to do that [share one card among multiple devices] with the Nintendo Switch, we haven't commented publicly on, and I'm not going to do it here."
So, yeah, who the heck knows. I would place my bet on it supporting something like download play, as I can't see any real downside to it. Maybe it's just another part of the big list of stuff they're waiting to announce three days before launch (for some reason).