Hi!
Trying to make the two wheel balancing robot with 2X SimpleFOC Boards stacked on an Arduino Uno R4. The MPU6050 works on the Arduino itself and on the first FOC board on top of the arduino. When I stack the second one on top of the first and plug in the MPU it does not.
Image is of the resistor soldering, which may be the issue.
Why do you think this is?
Are you using a4 and a5 for i2c sda and scl?
Can you measure resistance between
A4 - gnd
A5 - gnd
A4 - 5v
A5 - 5v
Measure before and after you add the board. My guess is that without shield they will all be open circuit (infinity), but with shield some will read a resistance, indicating pullup or pulldown. This might be affecting i2c bus.
On Arduino Uno, the SDA and scl pins are wired to the A4 and A5 pins. So you won’t be able to use those pins for encoder unfortunately. In my setup I usually used the A and B pins for
That setup is not possible with the shield, you’ll need to use the encodes pin A and B connected to the A and I of one shield and to the B and I of the second shield.
And make sure to have A5 and A4 not connected on the shield. 
