What is the term for the double strand of fiber produced by twin silkworms that join in one cocoon?

The term for the double strand of fiber produced by twin silkworms that join in one cocoon is known as Doupioni silk. This type of silk is characterized by its unique texture, stemming from the process where two silkworms spin their cocoons together. The resulting fabric is often irregular and has a nubby appearance due to the varying thicknesses of the strands, which adds to its aesthetic appeal. Doupioni silk is commonly used in luxury textiles, adding a distinctive flair to garments and home décor items.

Other types of silk mentioned, such as filament silk, fleece silk, and pongee silk, do not reflect the specific production method of twin silkworms but refer to different characteristics or processes of silk production. Filament silk is a more general term referring to the long strands produced by individual silkworms, while fleece silk typically refers to a type of silk that has been carded and spun, and pongee silk denotes a specific type of lightweight silk fabric. Therefore, Doupioni silk stands out as the correct answer for this particular question.