To create 2 parallel rows of stitching, especially as a hem for knits that duplicates the look of a cover hem machine, this is one of your two choices. The 4.0/75 Twin stretch needle has a wider space (4 mm) between the needles. The needles have a rounded tip as in the Stretch Needles - most suitable for knit fabrics.
Londa says: "Be sure to stabilize with stretch interfacing on the Hem ALLOWANCE and to lower your needle tension a bit to create a non-tunneled, nice look on your knit hems. See my knit DVD for many hints on stitching with twin needles: Stretching Your Knit Sewing Know-How.