Epoxy: adding fresh over cured
In my experience fresh epoxy will bond with already cured resin just fine.


Both the cured and fresh epoxy should be the same chemicals. I have no experience with layering different types of resin.
Given that the surface of the cured resin is clean (no dust or oily residue) and both batches have the same color you cannot see the individual layers after the fresh resin cured.
Given the first batch of resin cured right (the resin to hardener ratio was right and it's not still soft or sticky) I was never able to seperate individual layers of resin without breaking the object. You cannot "fix" a batch of resin that doesn't cure because of wrong mixing ratios by pouring more resin on top. You can only stabilize slightly bendy resin.
If you expect the joint to come under tension, you should roughen the cured surface with coarse sandpaper. This gives the fresh resin a bigger surface to bond to but will become invisible if you remove the dust before pouring. 
You should keep surface tension in mind. Pouring fresh resin over only part of a cured object creates a visible transition and the fresh surface will reflect light differently because surface tension pulls it into a different shape. I highly recomment covering the entire surface of a cured object with a thin layer of fresh resin to repair dents or holes. Pour enough resin that the surface tension cannot pull the fresh resin away from corners or high edges (happened to me several times)


When I craft with resin I often cover objects in several thin layers of epoxy. In videos you often see people grinding and polishing their epoxy jewelry to a shine in a long process. If you don't need perfectly even surfaces, you can pour a thin layer of epoxy on top instead and archive a finish that's clear and shiny without the trouble of polishing.