Need to DUPLICATE HTTP requests to two servers

What we chose eventually was using Gor (now GorReplay)

this solution allows installing a listener on the original host, and what it does is record any incoming HTTP request, this is done without modifying it or blocking the production server from handling it.

It then pushes these requests to a Gor replay server which can handle all kinds of useful logics of splitting/increasing load based on the incoming requests - you can send a percentage of requests to a dev server, or a multiplication of the requests to create simulated (but from real traffic) load on your staging environment, or both...

Sadly this is at the server level, so you have to install on each production server to get all the traffic, but you don't have to, and it provides a great solution for the problem outlaid in my question.