How to evenly distribute data to multiple GPUs using Keras
Check out the docs on TensorFlow GPU usage
If you wanted data parallelism where you run a copy of your model on multiple GPUs and split the data between them, you could use the tf.distribute.MirroredStrategy.
The tf.distribute.Strategy docs are also a good source to read.
Also, you should also profile your application; adding a second GPU has the potential to reduce performance depending on what your bottlenecks are.