[Python] Aggregating output from langchain LCEL elements

I have two chains, one that generates a document and one that creates a short document resume. I want to chain them, using the output from the first on inside the other one. But I want to get both outputs in the result.

Before LCEL, I could do it using LLMChain's output_key parameter. With LCEL, there seems to be a RunnablePassthrough class, but I don't seem to get how to use it to aggregate the output. Code example:

generate_document_chain = generate_document_prompt | llm | StrOutputParser()
resume_document_chain = resume_document_prompt | llm | StrOutputParser()

aggregated_chain = generate_document_chain | resume_document_chain
content = aggregated_chain.invoke({"topic": topic})

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