For something small, such as pens, CA will work. But with larger projects, use epoxy (CA doesn't have very high shear strength). The solid surface adhesive is high strength epoxy with the color dispersion added to match the particular finish.
For a better matched seam, sand the solid surface with a fine grit, and add the dust to the epoxy.
Edit:
Forgot to mention... we sand the finished side (smooth), with 220 grit and adhere those together for small projects. The finished side is already flat, unlike the back side, which has grooves for adhering to the substrate. In the process of sanding this side smooth, the surface can become distorted and no longer flat. That will result in gaps.