So basically, you have built up a technological debt and are now trying to pay it off. Sorry, but if I knew an easy solution to this I wouldn't tell you!
But, from what you've said so far, you might like to look at
ASP.NET Themes.
You would have to merge the html and then use the resources ( images and css ) from each theme to present the two UIs.
Without looking at your code, I don't know if this is the best solution for you. I just thought I'd mention it in case you haven't considered it.
Good luck :)
Nick