Hi,
My thinking is if one fan's rpms pick up appreciably when the other fan comes on that you might get more benefit with two fans as the single fan is loading trying to pull air through the tube.
Usually fans have CFM ratings on them. If you can find a number and do a search you might find a data sheet to give you the info you need. Then knowing the room area you could get a ratio of air moved to time and then calculate how long it takes for enough temp differential to take place to make it worth turning on the fan.
After a specific point it becomes a null. The air at the ceiling isn't warming fast enough for the air moved so you're just spinning your wheels (and fan) which consumes extra power so I would think balancing the system as best as possible is something to work toward.
Rob
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