Also, the Le Grand offers the 2-for-1 promo during the weekdays too (when you click on the above link and select weekdays for your stay). For some potential dates that I checked, weekends were sold out, but weekdays were fine.

The rate works for the classic rooms, superior rooms, and deluxe rooms with only ~20 euro/night difference between the classic and deluxe. It doesn't work for suites or room rates that include club access. I booked the deluxe at ~220 euro/night, vs ~200 for the classic. This still represents significant savings (though not 50%) over the best available rate otherwise found on the website, and is probably one of the best values for a 5* hotel in Paris right now.