If you are within the recoat time of the 2 part epoxy, it will be fine. Any longer and as with any 2nd coat, lightly sand for improved adhesion and again it will be fine. The one part urethane has a very similar chemistry to the 2 part epoxy, it just utilizes moisture in the air to complete its curing. The 2 part cures faster as the 2nd component is much more reactive. In terms of putting a one component coating on a cured surface, that is what you are doing anyway if you paint a fiberglass boat. Even though it is polyester chemistry, the boat surface is cured (crosslinked). It just has to be cleaned of wax and mold release then roughened up so the urethane coating will remain adhered.