This is exactly what I was doing right now: Two boxes vertically stacked in the same card. The top box is a plain TextArea with placeholder instruction to type or paste in their Markdown content (… I also included a URL to a free online Markdown editor - StackEdit); and then immediately below that is the RichText box for grabbing the above content and inserting it there for previewing. Is that basically what you did, too? (Confirming). Thank you.
PS: It requires a bit more event handling (to copy over the text), hiding and unhiding, etc, but at least I understant it.