import numpy as np

a = np.array([[1,2,3], [4,5,6], [7,8,9], [10,11,12]])
b = -a
c = np.reshape(np.array(zip(np.ravel(a), np.ravel(b))), (4,3,2))

print "a = n", a, "n"
print "b = n", b, "n"
print "c = n", c

np.dstack((a,b)).reshape((4,3,2))