Do you really want to use an Apache httpd to load-balance the 2 httpd? That looks like many steps for me.
I am also failing to see what's the advantage of this setup which adds an extra hop and doubles resources required for those requests. For setup with multiple balancers a typical use case is to use hardware load balancer or use DNS balancing.
I agree that's not a typical usage but there are some security policy on the project which force me to use separate apaches. Anyway we finally convice our client to use a hardware balancer to do that.
But I am still curious to know if it was possible to do it in some way.
Thanks for your answers.