As GTB says book individual portions separately. I do it ALL the time. Then if the weekday rate goes down, you can go online and grab the lower weekday price. Same goes for the weekend portion.

You can also have the reservations merged the two reservations into one, but then you have to cal every time there is a rate change; AND if the weekday rate goes down, but the weekend rate goes up, you will not be able to keep the lowest rate for each as the whole reservation will reprice at current prices. But as long as they remain separate reservations, then you have the best of both worlds.

When you check in, notify the FD that you have back to back reservations, and ask them to block the same room for your second reservation, or merge the reservations at that time.