Nginx load balancer replicate traffic to upstream server

I have nginx loadbalancers accept Post data and send to upstream application servers (Roundrobin) using fastcgi. Now, I would like to replicate traffic to different set of cluster. How do I do that in nginx?

Post traffic --> Nginx LB --> Cluster 1
...........................|
...........................v
....................Cluster 2

Cluster 1 and 2 should receive same events.

In my case user send post traffic once and Nginx should be able to replicate (replay) traffic to Cluster 1 and 2.