Both outlets get fuel from the same place inside the petcock. If one does not pass fuel, it can only be blocked very directly in the very short and easy to check passageway that leads to the nipple.
These petcocks (seemingly more especially so than most) have a very strong tendency to dispense fuel ONLY out of one outlet or the other when there is zero resistance to both nipples. In other words, with both hoses off, when you turn the valve open, fuel usually will pour ONLY from one nipple. Close that one off with your finger, and fuel will instantly start pouring from the other. It may seem like one is preferred over the other, but in operation both carbs get the fuel they need because as soon as one carb is full the other will fill as well. If you run clear fuel lines you will notice this is very often the case (presuming nothing else is wrong). You will also see that the fuel lines never get filled up to the petcock, and that the upper half of the lines is always empty. This is normal.
You can test the petcock very easily. Turn off the fuel valve. Simply pull both fuel lines off of both carbs. Blow air by mouth into one of the lines - does not matter which one. Air should come out of the other. If this happens, and fuel comes out of one of the lines when you turn on the gas, the petcock is fine. If you can not blow into the fuel lines, one of the lines or passageways is blocked. Unscrew the bowl and remove the screen. The passageways from the nipples are very simple and easy to inspect. You do not need to disassemble the valve. You can double check the valve while you have the bowl off simply by putting a cup under the petcock and turning on the valve. If fuel pours out, the valve is fine.
