Symbol 3775 from ISO 7000 - Title/Meaning: Double product; double sheet 
Function/description: To indicate that two sheets of material/substrate (such as paper) or two books or signatures are being processed, transported, etc. together at the same time.
Own work using: iso.org/obp/graphics/grs/7a897bce-d566-4e7a-8162-d3b0d6c4991a 200.png
Vectorization:  Mrmw