Looking forward to when HAProxy support for HTTP/2 lands since they refused to implement SPDY support.
Here's a list of common servers support for SPDY/HTTP2: https://istlsfastyet.com/#server-performance
Here's a list of common servers support for SPDY/HTTP2: https://istlsfastyet.com/#server-performance
The frontend definition advertises spdy and http/1.1 protocols via npn. (this should be now ALPN, HAProxy supports it)
The ssl_fc_npn ACL routes to the SSL-teminated traffic to the appropriate backends.
Nginx is configured to serve two backends with one port for each protocol. There can be multiple instances with round robin, if necessary.
This setup scales and is extensible for additional protocols
You can use the routing capabilities of nginx in the backend to break the traffic further down.