Invisible Seams are Tricky
There are many ways to seam crochet, whether you slip stitch or single crochet the edges together, or you sew it with a yarn needle. Depending on the edges to be seamed, seaming them together unobtrusively may be impossible—no matter how many different seaming methods you try.
Neck Warmer? 'Seams' Even Trickier!
Second, the stitches are going in two different directions. One edge is along the ends of rows. Normally this is great with Tunisian crochet. Tunisian row ends have a naturally seam-ready chained edge (especially when you do #4 in this list).
The other edge is one complete row (NOT along the ends of rows). In other words, the stitches in the two edges to be seamed are at right angles to each other. They're perpendicular.
The other edge is one complete row (NOT along the ends of rows). In other words, the stitches in the two edges to be seamed are at right angles to each other. They're perpendicular.
This seaming situation is really common with the type of cowl called a neck warmer. Slip Tectonics is a great example.
'Seamly' Star Stitches
Do you love it? I'm so glad I used a soft long-striping yarn so that I could pull out a color to use for the star stitch seam.
Now to do this with all the neck warming things!