You can use the same editor for more than one value in a server behavior like an update. In the update you would just specify that both columns get their value from the HTMLEditor instance on the page.
However, I do not fully understand how you would like this to work from the perspective of the user. Would you have a page that loads with two text areas on it, and if the user selects one text field the editor would show and be associated with that field, but if the user set focus to the other text field the same editor would show but be associated with that text field?