I think it's a great idea, and would vote for the series to be a week shorter to accommodate it. They could even have the semi in two stages, as was the three person final, with one couple going after the first dance leaving two to fight it out.
Failing that, a lot of people would opt for a week with no elimination, but I've never been sure how the format could work as there'd be no point in the results show. How about a spoof elimination? That is, the show is exactly as normal, they even do the dance off, but then as the judges are deciding they hit them with the news that no-one's going home that week. I'd love to see their reactions! You could only do it once though.