I used 3 strands of yarn at any given time: worsted or bulky and sock weight.

Long-tail CO 94 st.

P1, sl 1 for each row.
You will be working half of the stitches in each row, creating one side of a double-sided fabric.

Work in purl double knit until square.
BO 2 inches together
BO active stitches on the side, placing the “sl” stitches on a separate needle to create an opening until 2 inches from end.
Work in p1, sl1 to end of row.
BO 2 inches together and then BO remaining sts on second needle for second side.
Sew in invisible zipper to opening.