You can create a 3D array containing your 2D arrays to be averaged, then average along axis=0 using np.mean or np.average (the latter allows for weighted averages):
np.mean( np.array([ old_set, new_set ]), axis=0 )
This averaging scheme can be applied to any (n)-dimensional array, because the created (n+1)-dimensional array will always contain the original arrays to be averaged along its axis=0.