Washing Soda and Borax are listed as ingredients in many homemade cleaning potions and most notably in the homemade laundry detergent recipe that’s found all over the web. As a fabric artist and dyer I’ve had a good amount of experience with both of these items.
In laundry their cleaning effects are almost identical. They both add to the cleaning power of the wash. Both are water softening but not enough to solve a serious hard water problem. I can’t see any value in adding both. I can’t see any harm in it either.
Washing soda may lose it’s power in liquid form so if you’re making a liquid laundry detergent (or spray cleaner) you might want to use borax instead. (No commercial liquid detergents have washing soda in them. Some have borax)