I love that cardigan. It's in my Ravelry queue. I knit a sweater with a pattern that was similar and I used this Berroco how-to video
I don't think its going to show as a weird line of stockinette in the middle of your pattern. It might not be totally seamless, but people are used to seeing seams on ready made sweaters so I don't think it will look too weird. Maybe you could just try it with one shoulder and pull it out if you don't like it.