I work with several software packages (AutoCAD/Rhino/Sketchup/Blender) that all have "Mirror along line" capabilities or use plugins that do. I live by it.
Though I haven't been using Corel as much as I used to, I'm currently back using it. (2018) I seem to remember at some point I had a script or plugin that gave me mirror on line capabilities. Perhaps CADTools 5? Or maybe it was a how to video. Any suggestions?
I have a macro that can "mirror" selected content - either mirror the actual content, or duplicate the content and then mirror the duplicate just created. My macro does the mirroring using whatever mirror line the user defines; not limited to horizontal / vertical. The mirror line is persistent until you change it or destroy it, so you can keep using it to mirror additional content.
These are one-shot operations; not a dynamic "Keep updating the mirrored content whenever the original content changes". For that sort of thing, look to the native Symmetry capabilities of CorelDRAW 2018.
My macro does some other stuff, too: moving selected objects a specified distance in a specified direction (specified by the same shape that serves as the mirror line).
...and it looks like this: