Thread the needle with 1 ply of colour A and 1 ply of colour B together
Step 1 – Bring the needle and thread up at 'a'
and go down at 'b'.
Step 2 – Come up splitting the plies as shown.
Step 3 – Go down at 'c' as shown.
Step 4 – Swedish split stitch is made.
This stitch is an isolated stitch but it can also be used to fill larger spaces. It's a good replacement for seed stitch filling too.
You can alternate the shades as shown in the last image to create an interesting effect.