Putting two patterns on one piece of furniture usually goes wrong, and the reason it works here comes down to a single decision made before either cloth was cut.
Two patterns clash when their grounds differ. A floral on a warm ivory ground next to a stripe on a cool white ground will fight, however well the motifs are chosen, because the eye compares the background colour before it looks at the pattern. Get the grounds to agree and you can put almost any two patterns side by side.
So the two cloths for this diwan are selected together, laid against each other in daylight, and matched on their ground tone first - the pattern comes second.
Where each goes is then a practical decision. The stripe takes the seat, because the seat is the surface that wears and, on a long single cushion, a large floral repeat would have to be matched along its whole length and would still be sat on and hidden. The floral takes the back and the outer faces, which stay visible and take no wear.
The bolster is made in the stripe, its ends pleated so the stripes run into the centre button rather than across it.