What can I use to even out the stain on my stairs?

We pulled the carpet off the stairs and found oak treads. My husband rushed things and used HTP Walnut stain without surface prep. The result was two toned stairs where the old stain and sealer was done for the caps of the carpeted stairs. I have gone back over them with surface prep and sanded with 120 & 220 sand paper but I still see such a major difference where the old stain was. I’m scared to restain until I am confident that the end result will look even. Suggestions please?