Hello Mr. Dan.
Let me answer the simple question first .
No need to shut down the generator to refuel . You can keep the motor running as slow or as fast as you like .
I have never tried it but have given it a lot of thought and there will be a lot of devil in the details .
The state of the gasifier never stays the same pertaining to how dense or coarse the char and ash is in the burn tube and grate area . As the char and ash builds up and purge the amount of vacuum to pull a steady amount of gas from the gasifier will change. Sometime we may need 10 inches of vacuum to pull a given amount of gas from the gasifier and other times it may require 30 inches to pull the same volume of gas. In short if you pull two gasifers from one vacuum source one will over heat while the other goes cold . However you could put valves or a Y type butter fly so the operator could suck heavy on the lazy gasifier and less on the hot or coarse gasifier . This would take a lot of attention from the operator and would have to watch temps of each gasifier and keep them synchronized
One other issue of two small gasifiers vs one larger is with the expense of building the gasifier .The 55 gallon drums and seals work very well and are cheap. Even though a 9 inch fire tube is half the volume of a 12 inch tube one would come out cheaper to use a 55 gallon drum even if he could get by with a container a little smaller . In short two small gasifiers may take up twice the amount of bed space as one big one.
It has always worked for me to tune the gasifier to make clean gas with the vehicle working at minimum loads and let the high end work out where it falls, and if more power is needed add in a little gasoline. On the other side of the coin if a gasifier is tuned for heavy loads but run at minimum loads you will end up with tar and motor problems. ( a little gasoline is much cheaper than motors 
)
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