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The error message could provide more information about what went wrong. Some possible reasons why this error could occur are:

1. Inconsistent input shapes: If the two input layers have different shapes, the model cannot concatenate or add them together. Make sure the shape of each input layer matches the expected shape of the corresponding data.

2. Incorrect layer types: If one of the input layers is a 1D vector and the other is a 2D image, for example, the model cannot merge or concatenate them. Make sure the layer types match the input data types.

3. Name collisions: If the names of the input layers or other layers in the model clash with reserved names or other layers in the model, an error can occur. Make sure each layer and input has a unique name.

4. Missing inputs: If one of the input layers is missing from the model definition, an error will occur when attempting to use it. Make sure each input layer is explicitly included in the model definition.

5. Incorrect input order: If the order of inputs in the model definition does not match the order of input data when calling the model, an error will occur. Make sure to pass the input data in the correct order when calling the model.

To troubleshoot this error, inspect the error message and the code surrounding the model definition to identify which of these issues might be causing the problem.