How to calculate the difference between two 3-axis accelerometers

Other Hardware Sensors
1

I have two devices with a 3-axis accelerometer each. Both devices are placed on a bigger object so they move together. The movement is quite slow, I expect a bandwidth < 1 Hz.

I want to calculate the difference in orientation between the two devices. But since they are positioned randomly on the bigger object how I can do the math?

Example of meta-code:

typedef struct
{
    float x;
    float y;
    float z;
} acc_t;

typedef struct
{
    float yaw; 
    float pitch;
    float roll;
} pos_t;

acc_t acc1;
acc_t acc2;
pos_t pos;

do_measure(&acc1, device1); // reads the accelerometer and fill the struct
do_measure(&acc2, device2);

do_diff(acc1, acc2, &pos); // calculate the orientation of acc2 compared to acc1

I have no problem to acquire the data from the accelerometer (do_measure()), but I don't know how to implement do_diff(). I'm not asking to write the code, but just a hint about the math behind.

2

An accelerometer can measure pitch and roll only. This tutorial explains how: How_to_Use_a_Three-Axis_Accelerometer_for_Tilt_Sensing-DFRobot

To measure yaw requires a North reference.