import numpy as np

nx=2
ny=3
nz=4

a=np.zeros((nx,ny,nz))
b=np.zeros((nx,ny,nz),dtype=np.int64)

for i in range(nx):
    for j in range(ny):
        for k in range(nz):
            a[i,j,k]=100*i+10*j+k
            b[i,j,k]=-(100*i+10*j+k)

f=open('datafile.dat','wb')
f.write(a.T.tobytes())
f.close()
f=open('datafile.dat','ab')
f.write(b.T.tobytes())
f.close()