This post is about the only thing I find on that subject with Google. Must be pretty small If there isn’t even links to buy it out there.
Connecting HA to multiple MQTT Brokers.
WTH- why can't I setup 2 (or even more) MQTT brokers?.
Local AND remote MQTT broker?.
Mossquitto get data from another MQTT Broker - lollipop camera.
What is the preferred MQTT setup?.
Here’s an external hardware vendor setting up a bridge between HA and themselves, the complete set-up… Home Assistant broker setup with SolarAssistant.
For more info on the same topic. All pretty much say the same thing. Multiple MQTT brokers running on the same server is pointless because you can bridge them together in very granular ways, there is no point to it.