There has been ongoing work in HAProxy for many years to support zero downtime reloads via a few different potential mechanisms (file descriptor passing, a socket server, etc ...). Unfortunately it turns out that it is really hard given the architecture of HAProxy. That being said, I'm sure patches are always welcome.

The post does mention that they did consider something similar to what you are suggesting with the multiple HAProxy instances but decided against it due to engineering uncertainties. Could be that they just overestimated how hard it would be.