One way to finish the stomacher would be to sew and assemble the stomacher just like the stays, and then add a floating lining at the end.  You can absolutely do that, and it’s the best method if you are hand-sewing your boning channels.  However, I prefer a different method.  

Here are some techniques to make marking and sewing the boning channels in the Scroop Patterns Cassandra Stays easier!

Testing with your assembled layers also gives you one final chance to make sure that your chosen fabrics are the right strength.  If they feel too weak at this point you can always add one more layer of buckram or interlining before sewing the boning channels.  It’s very hard to add an extra layer that provides any strength once the channels are sewn!