Вам нужно tf.keras.layers.Concatenate
:
input_layer = tf.keras.Input(shape=(28, 28, 1), name='img')
conv_1 = tf.keras.layers.Conv2D(16, 3, activation='relu')(input_layer)
conv_2 = tf.keras.layers.Conv2D(32, 3, activation='relu')(input_layer)
conv_merged = tf.keras.layers.Concatenate()([conv_1, conv_2])