You are absolotely right that my previous generation of hardware had an option where you could connect both ThermIQ and Thermia online. This used a a couple of low hw tricks that the old platform could support.
The new generation, with MQTT support, is based on a completely different platform which unfortunately does not provide the hw tricks needed. Connecting both online and ThermIQ will cause intermittent misses in data.
In short, there is no feasible way to do this with ThermIQ-mqtt