The Union tool combines two data sources with the same layout. Each input source is fed downstream one after the other.
Union has one input connector, but you can attach multiple input streams to it. Each input attached must have the same schema. Union has one output connector. Connect an input step by left clicking on the input tool’s connector and drag onto the Union tool’s input connector. Connect an output step by left clicking on the Union tool’s output connector and drag onto a downstream component's input connector.