Carlo Munier is one of the important composers for the mandolin. In his book “Il Nuovo Stile dei Duetti per mandolino solo, Op.278 ” he has studied the possibilities of the duo style for the mandolin. I have now found a video of the 1a. Fantasia from this collection, a great performance by the Japanese mandolinist Kiyosumi Orii.
The sheet music for this book by Munier is available as free doenload in the Petrucci Library (IMSLP):
Munier uses two staffs for the notation of this fantasia to make clear that it sounds as if two mandolins are playing. Here is the beginning of the Fantasia.