Yes, in theory, you should be able to get the silhouette that you want, so long as you're not starting with skirts that are really full. If the seam down the back of the one dress is on the straight grain and not flared than it sounds like you just need to deal with the side seams on both dresses. Where you will get a bit of difficulty is the pockets which may need to be unpicked out. Maybe your best bet is to hand baste down the seams to see if you like the effect and if it doesn't cause any diagonal drag lines. You see, straighter skirts usually have a bit of a dart at the waist line to create tummy shape, like on an ordinary straight skirt and without those you may not get a sleek enough fit. Give it a go with basting first and just keep trying a little more each time to get narrower.