with tf.device(gpu[0]):
    print("Using GPU: {}".format(gpu[0]))
    a = tf.random_normal([2000, 5000])
    b = tf.random_normal([5000, 1000])
    res1 = tf.matmul(a, b)

with tf.device(gpu[1]):
    print("Using GPU: {}".format(gpu[1]))
    a = tf.random_normal([2000, 5000])
    b = tf.random_normal([5000, 1000])
    res2 = tf.matmul(a, b)

sum_res = None
with tf.device(cpu[0]):
    print("Using CPU: {}".format(cpu[0]))
    sum_res = tf.add(res1, res2)