You will need to double the sweetener, because using a combination of two sweeteners actually doubles the sweetness. So if a recipe calls for 1 cup of Splenda, if you use a combination of Splenda and another sweetener (I use xylitol, and find the taste to be far superior to Splenda alone), you would only need a total of half a cup, ie a quarter of a cup of each sweetener.

HTH

Rosebud