This "Y" adapter duct is typically used when one in-line exhaust fan is to draw air from two different locations in the same room, or even from different rooms. Multiple sizes are available.
Dimensions (FY 4): 4" x 4" x 4" (12.5" wide, 10.5" long)
Dimensions (FY 6): 6" x 6" x 6" (13.5" wide, 12.5" long)
Dimensions (FY 8): 8" x 8" x 8" (14.0" wide, 10.0" long)