It is definitely a two step stamp...
The two large flowers are meant to be a two step. Same with the two small flowers. Stamp the more solid flower with a lighter color, then use your stamp-a-ma-jig to stamp the highlight stamp over it with a darker color.
Then there are two places for flowers on the stem stamp. With the sepals on the top place, the small flower goes there. The large one fits at the end of that lower stem.
You could also put the larger one on the top space by cutting one out and placing it there, and a smaller one can go in the lower space. They did that in one of the examples on page 79 of the catty.
Take a look at the gallery:
Stampin Up! Gallery: Field Flowers - Page 1 - Cards & More - Splitcoaststampers You will get great ideas there.