In a previous tutorial I walked through the progression of accessing a database in Java and Spring. In the comments for that video I was asked how to use the JDBC Client when you're using multiple data sources. I thought that was a great question and this tutorial is to walk through that scenario.
Create a new application at start.spring.io with the web,jdbc, and h2 dependencies or by clicking on this link.
There is a really good section in the Spring Boot Reference Documentation on Configuring Two DataSources.