A:Answer If you're talking about a multiplayer game like Super Smash Bros, then all you need is a second pair of joycons and you're good to go. You really only one game to play it localy as one Switch can pair with up to four pairs of joycons. Of course, that's providing that neither is playing any Singleplayer modes.
If you're talking about a single player game like Splatoon 2 (It's a multiplayer focused game, but it only allows one person to play it at a time per device,) then if they want to play at the same time they'll each need a Switch, and a copy of the game, Cartridge or Digital Download from the Nintendo E Shop. And if they want to play together or against each other, then they'll also have to be near each other for local wireless, or connected to the internet, which requires a subscription to Nintendo's Online service.
It really comes down to what games they're playing. Really the only other things you need for the switch is the Dock and an HDMI cable to the TV, both of which are provided right off the bat with the switch itself.
