Mix and match, and re-wear!!!
I was just thinking about this dilemma in a hypothetical manner when we got off our 8-night cruise to Acapulco Nov. 1.
We almost had to bring another suitcase because of that one extra day, and then I had the smart idea of rolling up the suits, and voila, made it! (Just a garment bag, one big suitcase, one small 20-inch one, a backpack and a tote.)
What I noticed was that all of our dinner clothes had a second wearing in them (we bring one suit and two pairs of dressy type pants, and mix them with all new shirts). The casual clothes were getting a little on the tired side, but they had a laundry special on the ship for $15 and you fill the bag.
I think you should be able to do the laundry for under clothes, Tshirts and such.
Also, and a big point, you are in fact on two different cruises with 2 different sets of passengers, so it's unlikely someone will recall what you wore 10 days ago to dinner.
Hope that helps somewhat.