WPEC 3.8.8 changed the id of the additional description DIV, so the plugin didn't work with it. I've fixed this, added two bits of CSS so that it looks nicer, and did a minor change to the JS. My changed version is attached below.
However, I noticed another problem for which I have no idea how to solve. It's this: if the main description area (not the additional one) of a product edit is in "Visual" (WYSIWYG) mode, then you click the "HTML" tab, then close the window/go somewhere else, then open a product for edit (or add a new one), this edit window already comes in "HTML" mode, as WP remembers the last mode you used, the same strangely also applying to the additional description area. Then, if you click the "Visual" tab to change to WYSIWIG mode, it comes empty, without the TinyMCE bar nor any area available for typing.
The workaround is simple: reload the page. It then comes in WYSIWIG mode with everything working. But as soon as you switch to HTML mode and leaves, it happens again.
Well, here's the changed code in
And here's the full version: