If you mean using the Blau's in the front and adding a different brand/size in the back, then it should be fine (actually common). Just check on the Ohms of the speakers so you don't over heat your power source (amp or head unit). This is assuming the power source has rear channels. If not, you'll need to be careful with how you wire the second pair. Running them in parallel (the positives together for each side and the negatives together on each side) will lower the resistance (Ohms). Running the speakers in series (daisy chain) will increase the resistance. The link will help explain series vs. parallel. good luck
