Can I vent a natural gas water heater and a natural gas hot water boiler out the same chimney from basement?

Sounds like you are asking if you can use an exhisting brick furnace to vent both appliances . Yes you can combine the vents but you must line the chimney with a metal liner first to meet code . If you dont line the chimney the water vapor in the vent gases has is very acidic and starts to eat the mortar in the chimney and that can cause the chimney to eventually fail . Make sure to size the vent for the combined BTUs for the 2 vents .