If you are using Xandrox 5% and spironolactone 2%, apply the spironolactone first, allow to set in a bit, then drip on the X.
If you are using Xandrox 5% and spironolactone 5%, apply the Xandrox first, allow to dry, give it a good 20 mins, and then apply a thin layer of the spironolactone cream.
If you are using Xandrox 15%, I would only use spironolactone 5% in conjunction with it. Drip on the Xandrox, allow to dry just a bit, and apply a thin layer of the spironolactone cream.