No problem.
Cuttlebone floats, but if not eaten it will eventually become waterlogged and sink. Members feed different amounts and sizes depending on their turtles---some love the stuff and will nibble on a whole piece if put in the tank, others prefer smaller pieces. Others will eat it in a more leisurely manner; some don't really take to it that much and it winds up sinking to the bottom. My turtles eat it more readily at certain times than others, and I think it depends on if they need more calcium in their diets.
In my case, my turtles prefer the smaller pieces, usually about head-size, and I will cut up a number of pieces and keep them in containers so I don't have to cut them so often (it can get messy, and I've poked myself with a sharp knife more than once).
Whichever way you try (one piece or smaller pieces) you need to take off the hard backing which is on one side. I use a sharp knife to pry it off, which usually causes the piece to break up.
I usually keep several pieces floating in the tank, so they can be nibbled on at will. If they're going through a phase where the cuttlebone isn't really being eaten, I cut back a bit. Your RES might not take to it right away, it would be good to give it a try.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-