1.js

async function mergeResponses(responsePromises, headers) {
  const readers = responsePromises.map(p => Promise.resolve(p).then(r => r.body.getReader()));
  let doneResolve;
  let doneReject;
  const done = new Promise((r, rr) => {
    doneResolve = r;
    doneReject = rr;
  });
  
  const readable = new ReadableStream({
    async pull(controller) {
      const reader = await readers[0];

      try {
        const {done, value} = await reader.read();
        if (done) {
          readers.shift();
          
          if (!readers[0]) {
            controller.close();
            doneResolve();
            return;
          }
          return this.pull(controller);
        }

        controller.enqueue(value);
      }
      catch (err) {
        doneReject(err);
        throw err;
      }
    },
    cancel() {
      doneResolve();
    }
  });

  const response = await responsePromises[0];

  return {
    done,
    response: new Response(readable, {
      headers: headers || response.headers
    })
  };
}

future.js

// This depends on transform streams & async iterators

async function mergeResponses(responsePromises, headers) {
  const {readable, writable} = new TransformStream();
  
  const done = (async function() {
    for await (const response of responsePromises) {
      await response.body.pipeTo(writable, {preventClose: true});
    }
    writable.getWriter().close();
  })();

  return {
    done,
    response: new Response(readable, {
      headers: headers || (await responsePromises[0]).headers
    })
  };
}

@PRMSA it allows responsePromises to be a mix of Promise<Response> and plain Response.

future2.js

const mergeResponses = async (
  responsePromises = [Promise.resolve(new Response())], 
  headers = new Headers()
) => new Response(new ReadableStream({ async start(controller) {
  for await (const response of responsePromises) {
    // Bonus example: controller.enqueue(await response.text());
    await response.body.pipeTo(new WritableStream({
      write: (data) => controller.enqueue(data)
    }));
  }
}}),{
  headers: headers.has("Content-Type") ? headers : (await responsePromises[0]).headers
});

// usage example
globalThis.onfetch = (event) => event.respondWith(mergeResponses([...responsePromises]));

The function parameters got set with empty default values for typeInferencing in IDE's
so the above example is 100% Typed