Not really sure- I have a chart around here somewhere for tank pressure orifice sizes. That connection is certainly not meant for high capacity, but it should be able to flow over 50k btu/hr, but you are correct- more than one furnace along with a water heater would most likely exceed the capacity. The correct way is to buy the 4 port adapter.
I looked around to see if I could find capacities for those "steal saver" adapoters that let you hook a 1 lb cylinder to a large grill, but could not find anything.
