Which hose appliance combines two hose lines into one?

The appliance that combines two hose lines into one is known as a Siamese. Typically used in firefighting operations, the Siamese fitting allows firefighters to connect two separate hoses, often from different sources, and merge the water flow into a single line. This is particularly useful when handling larger volumes of water or when the available water supply is limited, as it enables a more effective and efficient response to a fire incident.

While other options like the WYE and splitter may also involve the management of hose lines, they serve different purposes. A WYE splits one hose line into two, allowing water to flow through both lines simultaneously rather than combining. A splitter, on the other hand, distributes the flow from one line into multiple lines. Therefore, the Siamese fitting is specifically designed for the task of merging two hoses into one, making it the correct choice for this question.