Multiple Transaction Managers in Spring Boot for different EntityManagers

I need to connect to two different databases from a single application. The trouble is that my appEntityManager does not have a transaction manager associated with it and I am not sure how to do it. The @Primary adminEntityManager is able to use the one provided by spring boot without any trouble as described here.

The configuration above almost works on its own. To complete the picture you need to configure TransactionManagers for the two EntityManagers as well. One of them could be picked up by the default JpaTransactionManager in Spring Boot if you mark it as @Primary. The other would have to be explicitly injected into a new instance. Or you might be able to use a JTA transaction manager spanning both.

I have annoted the configuration with

@EnableTransactionManagement

And here is the relavant beans

@Bean
@ConfigurationProperties(prefix = "datasource.app")
public DataSource appDataSource() {
    return DataSourceBuilder.create().build();
}

@Bean
@Primary
@ConfigurationProperties(prefix = "datasource.admin")
public DataSource adminDataSource() {
    return DataSourceBuilder.create().build();
}

@Bean
public LocalContainerEntityManagerFactoryBean appEntityManagerFactory(
        final EntityManagerFactoryBuilder builder) {
    return builder
            .dataSource(appDataSource())
            .packages("au.com.mycompany.app.bomcommon.domain")
            .persistenceUnit("appPersistentUnit")
            .build();
}

@Bean
@Primary
public LocalContainerEntityManagerFactoryBean adminEntityManagerFactory(
        final EntityManagerFactoryBuilder builder) {
    return builder
            .dataSource(adminDataSource())
            .packages("au.com.mycompany.app.bombatch")
            .persistenceUnit("adminPersistentUnit")
            .build();
}

//I thought this would do it but I am getting an exception
//No qualifying bean of type [org.springframework.transaction.PlatformTransactionManager] is defined: expected single matching bean but found 2: appTransactionManager,transactionManager
@Bean
public JpaTransactionManager appTransactionManager(@Qualifier("appEntityManagerFactory") final EntityManagerFactory emf) {
    JpaTransactionManager transactionManager = new JpaTransactionManager();
    transactionManager.setEntityManagerFactory(emf);
    return transactionManager;
}