You could do a couple of things ...
a. You could always write a query that can "read" from two different tables. You could map the result from these two queries to an object or a composite object containing two objects you want to map to and finally write it out. Just usual usage - nothing different required
assumption: both you tables are in the same database and can be access via a join query
OR
b. You could use a composite Item reader. Here is an example of how that could be done.
https://github.com/langmi/spring-batch-examples-readers/blob/master/src/main/resources/spring/batch/job/readers/jdbc/jdbc-composite-item-reader-job.xml