Can this be used for an interior concrete stain

That will depend a lot on exactly the type and makeup of the stain. A water-based, latex type stain is not recommended in the Double Duty as they will tend to be too thick. Most oil-based stains can be sprayed with the Control Spray Double Duty without any thinning needed. Make sure the material is something that will clean with either water or mineral spirits. If it requires a harsher solvent for cleaning, it should not be used in the Control Spray Double Duty.