Well, here's what I think, after spending an hour with this pattern stitch:
The numbers given for the pattern (18 + 5) are wrong.
Cast on 37. (although this is not a multiple of 18+5)
Every RS row begins with the 37, ends with 41.
Every WS row begins with 41, ends with 37.
But adding another span of the pattern stitch is beyond me. I don't think you cast on 37, place stitch marker, cast on a further 37.
Tomorrow I'm going to print another chart or two and do a cut/paste together for a double wide span of pattern stitch and see how it looks.
Simply casting on 37+37 for a double wide version is going to look wonky where they meet in the middle, I believe.
There is a piece to this puzzle that we're all missing, and the webpage is
keeping the secret to themselves.
Edited to add:
Mystery solved! The cut and paste worked. I got the picture.
(I stand by my first impression. Forget the 'multiple of 18 + 5. Bah!)
Two motifs wide: cast on 37.
Three motifs wide: cast on 55.
Four motifs wide: cast on 73.
Five motifs wide: cast on 91...etc etc, adding 18 for each extra motif.
These cast on numbers don't include edge stitches.
For the four motifs wide, your Row 1 would look like this:
K5, K2tog; work the repeat area 3 times; end with YO K1 YO K3 YO K1 YO SSK K5
The 'repeat area' is designated in the written directions here.
Of course, the repeat area instructions change as the rows progress. I'm only discussing the very first row of the repeat. But if you print a chart that I've suggested, you'll be right on track for Rows 2-12. The chart won't steer you wrong, or be confusing. If you're not a chart knitter, now is the time to learn to work from a chart.
Will take a photo of my cut and paste modified chart later.