reduce even array in half by merging columns

Question

Assuming i have the following array:

a = array([[[4, 8, 7, 3, 1, 2],
        [3, 1, 8, 7, 1, 9],
        [0, 0, 3, 0, 7, 6],
        [1, 1, 5, 0, 5, 1],