Regardless of our personal opinions, this is an incredibly generous offer by IHG standards.

Your best bet is to accept & use the 100,000 points for the second room. And then pay to upgrade the room to a higher spec if you feel the room isn't good enough.
Or you can use the Chase free nights again to secure the second room. Convert the 100,000 points to £225 and use this money to upgrade each room by a tier (Not the best use of points in general but if you don't want to spend any money, it's free via reselling gift cards).