I am sorry you are having trouble using the DRV8838 motor driver. It is difficult to tell what might be the issue without additional information, and this comment section is not very well suited for troubleshooting. Can you post a more detailed description of your setup including pictures of all your connections on our forum?
I am sorry you are having trouble using the P*. However, the blog is not an appropriate place for troubleshooting your problem. Instead, you might post this on our forum with more details about your setup.
I am sorry you are having trouble controlling your servos; I think some people on the Pololu forum might be interested in taking a look. I recommend posting to the Software and Microcontrollers section, and including your code as well as oscilloscope traces.
It is possible to use more than one dual VNH5019 shield on an Arduino, but it would require a lot of changes to get it to work. You would have to remap the control pins, do some additional wiring, and write your own code to set up PWM on the correct pins for all of the shields.
You can read more about how to make some of those changes inside the "Remapping the Arduino Connections" section of the VNH5019 shield's user's guide, which can be found under the Resources tab of its product page:
By the way, we break out the connections to the motor drivers along a single side of the shield, so if you do not need to stack the shields, you could just make your logic connections there. This might make using both shields easier.