Factorio Mod which adds double furnaces to the game.

The double electric furnace looks similar to an electric furnace (but it's green) and it can smelt iron ore directly to steel (but can't smelt anything else).
Also includes a 2x2 double steel furnace.

Updated to 2.1 and beyond as original author is no longer around.

Original for 2.0 and earlier: Double Furnace