In the settings, the option for the portals is named "Enable portals". Once you have that checkmarked, you can drag food onto any other character, and it will show a line. Dragging the food above the line will put the food in the stomach, while below the line will put it in the bowels.
No need to go into the game files or anything of the such. If the option isn't present, then you may simply need to redownload the game. (Tested it myself a couple of times, the option is present on both the web version and the downloaded version)