Doesn't matter, mix 'em up all you want. The main reason to do so would to balance a certain kava out in some way that you prefer.
For example, you bought a bag that is weak, but you also have a bag that is strong: add the strong one to the weak one to make it more effective...or vice versa.
Add a body kava, to a heady kava.
Add palatable kava to a repugnant kava.
Nope. Mix away...lots of mix kava to try and change the effects (more headiness, more heaviness, taste better etc.). It is like coffee or wine, some of the best are blends.