Not exactly sure what you mean. 2 Players who play on the 3DS version will still be able to play Terraria together after the Switch version comes out.
If you're wondering if a Switch player could play with a 3DS owner, that isn't the case. Ontop of being different hardware, the Switch version will be a different version compared to the 3DS version, so they won't be compatible. As far as I know (I could be wrong on this part) The Switch isn't capable of playing wireless with 3DS owners like how some games for the Wii U & 3DS supported playing Multiplayer games on those platforms. Likewise, an Xbox 360 player can't play together with an Xbox One player, even if they have the same game, since it's different hardware and the games are built differently to support the hardware they run on.