How to use output of Indirect Kalman Filter(=error state kalman filter)
In this article they use a Kalman filter to estimate the bias of the rate gyro and then subtract it from the true rate but they are running the states with only rate gyro info:

so their rate states are: 

$$ x_k =\begin{bmatrix} \theta_k \\ \dot{\theta_k}  \end{bmatrix} $$

then they subtract the bias from the new rate:

rate = newRate - bias;
angle += dt * rate;


In this other article when they combine the two they use a fraction of each the accelerometer and rate gyro. 

 angle = 0.98 *(angle+gyro*dt) + 0.02*acc 


Hope this helps. 

One thing I'd like to add is this works for a 6DOF sensor, but its better to have add a magnetometer so then you have a solid (well as long as you can trust magnetic north) sensor updating your rates.