In reality, you only need one. You can for sure add more and it is like having another chamber to filter more and cool more, BUUUUT, the more weight you add, the more risk you take by adding stress to the glass. This can cause a joint to break off, so we recommend only one ash catcher per water pipe. If you want more percolation to think about a larger water pipe.
If you really want to have a clean water pipe and REALLY taste the herb as you have never tasted it, you should think about a vaporizer like the Silver Surfer. There is 80%-90% fewer tars as well as when you eliminate combustion you nearly double the active ingredients as combustion destroys up to 60% of the compounds. Another thing to note is that vaping is up to 80% bioavailability while smoking is only 30%.
Learn more about smoke vs vapor