not sure if i am understanding you correctly, are you are trying to use points for the first night and use additional points that night to confirm a specialty room- in this case the towers- and pay cash for the second night in the regular hotel and hope for an upgrade to the towers because you are already in the towers? if so, i would tell you not to expect anything outside the rules which is what you would be hoping for as that is not in the mentality at the hotel. if this is your strategy though you should do this and then you can upgrade the paid night to a towers room for 2500 points in advance or 1500 points if available at check in. this may be the way to go. by the way 1500hkd is not a bad rate these days at the hotel.
So, for best chances at a Towers room for 2 nights when using cash one night and points the next, is it better to book the first night with cash and the second with points, or vice-versa?