I need to power 4 actuators, and it looks like 2 stacked Pololu Dual VNH5019’s will let me do that. I’m trying to figure out though, is it possible to share the 12v power for this setup, either by wiring the GND/VIN of the two shields together or splitting the power supply?
I’d really rather not have to have 3 power supplies - one for each shield and one for the arduino!
Yes, you can power both VNH5019 shields using a single 12V power supply as long as it can source enough current. Most Arduino boards are rated to have an operating range of 7V to 12V, so if you are using a 12V power adapter, it would probably be fine to use that to power your system. However, if you are using a 12V battery, keep in mind when fully charged, the battery can exceed the recommended input voltage (12V) for most Arduino boards and possibly damage it.
By the way, if you want to control each shield independently, you might need to remap the pins and control the shields separately in your code.