For the second format, you could use column_stack
followed by ravel
In [8]: img1 = np.arange(5, dtype='float32')
In [9]: img2 = np.arange(5, dtype='float32')
In [10]: np.column_stack((img1,img2)).ravel()
Out[10]: array([ 0., 0., 1., 1., 2., 2., 3., 3., 4., 4.], dtype=float32)