I have not found a way to do this with LangChain, but I found a function that allows me to flatten the output and results in what I want, although it seems a bit clunky and I believe there must be a better solution.

The key is to add the following function to the chain:

def flatten_dict(*vars) -> dict:
    '''
    Flatten a dictionary by removing unnecessary mid-level keys.
    Returns a Runnable (chainable) function.
    '''
    flat = {}
    for var in vars:
        keys = [k for k in var]
        for key in keys:
            if isinstance(var[key], dict):
                flat.update(var[key])
            else:
                flat[key] = var[key]
    return flat

chain = (
    {"first_prompt_output": first_chain, "possible_values": RunnablePassthrough(), "first_value": RunnablePassthrough()} 
    | RunnableParallel(result={"second_prompt_output": second_chain, "first_value": itemgetter("first_value")})
)
| flatten_dict