Spring integration flow with two sources

I try to create a flow which get message from 2 sources (1 mqtt and one from a user service interaction) and it generate message to another mqtt. In fact, i try to use this answer: How to crate Spring Integration Flow from two MessageProducerSpec?

And here is my result:

@Bean
public IntegrationFlow mqttInFlow() {
    return IntegrationFlows.from(mqttInbound())
            .channel("mainMessageChannel")
            .get();
}

@Bean
public IntegrationFlow mqttTestMessageFlow() {
    return IntegrationFlows.from(messageService.testInbound())
            .channel("mainMessageChannel")
            .get();
}

@Bean
public IntegrationFlow mainMessageFlow() {
    return IntegrationFlows.from("mainMessageChannel")
            .handle(eventServiceHandler())
            .split(operationSplitter())
            .handle(mqttOutbound())
            .get();
}

But i have the following error:

java.lang.IllegalStateException: 'outputChannel' or 'outputChannelName' is required
    at org.springframework.util.Assert.state(Assert.java:70)
    at org.springframework.integration.endpoint.MessageProducerSupport.afterSingletonsInstantiated(MessageProducerSupport.java:136)