sound reduction in bathrooms
In order to deaden the sound, you need to figure out which bathroom you want to rip down the drywall and add insulation. An empty wall is equivalent to a drum and will simply transfer sounds from one side to another. Fill it with fiberglass insulation or rockwool.
After you put up the drywall, you can consider adding another layer. If you want to really kill the sound, google for a product named GreenGlue and consider placing between layers of drywall.
NOTE: This is the method used for home theaters...