All versions of Riptide GP: Renegade support split-screen local multiplayer, even the mobile and tablet versions.
Unlike online multiplayer, which works over a network connection, split screen local multiplayer is designed to work on a single device. The screen is divided according to the number of players, and each person gets their own small screen.
Split screen local multiplayer REQUIRES a separate gamepad controller for every player. Although it does work on phones and tablets, it is really designed primarily for people playing on a TV, with a nice big screen.
On Nintendo Switch, you can play split screen multiplayer in docked and tabletop modes. You can play with any combination of Joy-Con controllers and Pro controllers.
If you want to play together, each with your own mobile device for example, you should play "Online Multiplayer" and connect over a network.