Port a Nintendo DS game to Steam

Yes, if you made the game or hold a license. You rebuild it in a modern engine and export a desktop build for Steam. Summer Engine generates playable scenes from a description of the original, lets you edit in C++, GDScript, or C#, and exports Windows, macOS, and Linux builds.

Side by side is usually best on widescreen, because DS games often used both screens for live information at the same time. A combined single view with the touch content as HUD also works. A toggled overlay only works when the second screen was consulted occasionally rather than continuously.

Yes, for pointing, dragging, and selection. The gaps are multi touch gestures, actions that required the stylus and buttons at once, and microphone input. Every one of those needs a designed alternative, plus a full gamepad path so controller players can finish the game.

Use integer scaling. 4x gives 1024x768 per screen and 5x gives 1280x960, both of which keep pixel art sharp. Non integer scaling looks blurry on DS era art. Fonts and dialogue boxes almost always need re authoring at the larger size.

For grid based, puzzle, and precise platforming games, yes, keep eight directional digital movement. For action games, analog stick input feels better but changes tuning, so add a deadzone and snapping where the design depends on exact directions.