is it possible for multiple aws lambdas to handle a single http request?

Unfortunately as you’ve found only a single Lambda function can be invoked, this becomes an orchestrator.

This is not ideal but will have to be the case if you want to use multiple Lambda functions as you’re serving a HTTP request, you can either use the Lambda to call a number of Lambda or instead create a Step Function which can can orchestrate the individual steps. You would still need the Lambda to start this, and then poll the status of it before returning the results.