Zoned Rocket Mass Heater?

Has anyone done or seen a "zoned" rocket mass heater: e.g. one that can exhaust via one of multiple pathways?
We're thinking it would be doable (maybe) to have two zones in a greenhouse system: one to heat 100+ gallons of water for watering plants and maybe to hot tub in, and one to heat a 3-4 ton bench of cob.
I would think we could use a "gate" like in a dust collection system where we could heat the cob bench OR the hot tub, but not have to do both. Probably we'd need 2 gates to open one zone (flue) and close the other.
The main issue I can think of would be potential for gasses escaping at the gate into the space. Is a flue in positive pressure - pushing - or negative, pulling from vertical flue (ours will be 20" feet (and insulated for the last 8' which should provide good "pull").
Gates could be sliding sheet metal or the like. Or even typical stove pipe dampers. Thoughts?

If you can bring them back to a common chimney/exhaust it should work. If you have two separate exhausts, you could have one try to draft all the time and overpower the other.

If cores were easier to build, I would say just to build two stoves. Two 6" stoves aren't that much more material cost than one 8".