How would I align text from one editor with text in another?

Imagine two editor boxes side by side. The editor on the left has text in it, the one on the right has nothing.

I would like to be able to highlight a point in left hand text box and add text that will appear in the right box at the same position as the highlighted text. If text on the left were to move up or down, so should text on the right.

Any ideas how I would achieve this?

Thank you.