Originally Posted by
Utopian
Older coticules, and some of the new ones, are backed with BBW (Big Beautiful ...just kidding! It's Belgian Blue Whetstone). That BBW can be naturally adjacent to the yellow coticule layer and is cut to leave those layers together. These are considered to be natural combination coticules. The majority of mine are of that type. I believe that both of "your" coticules are of that type. Certainly the thicker one is but I don't recall the other one.
Alternatively the BBW can be glued to the yellow coticule layer. The reason that the BBW is glued to, or left attached to, the coticule layer is that it provides strength or support to the relatively fragile coticule.
Many modern (recently mined) coticules are backed by slate. It is there for support only and is not considered to be of use for honing, but your results may vary.