one of the two petcocks on my 1976 xs500 leaks. can i just remove or block it off and run the one good one to both carbs without a problem? or should it be replaced? does anyone know why there is even two of these to begin with?

You can get a new gasket (the one behind the lever...if thats where the fuel is leaking thru) from Yamaha (#137-24523-00-00) or check on eBay or with Speed and Sport Yamaha. The other gasket & oring should also be availiable from the same sources.

Perhaps there are 2 shutoff valves because the construction of the tank doesn't allow the fuel to easily flow from one side to the other.

Keep both. My Buddy's Kawi KZ400 only has one petcock, by design, and it will run out of fuel before the tank is empty since gas can't pass from one side to the other when it gets below a certain level. I think we tested it and there was a good .75 of a gallon that didn't make it to the side with the petcock. Maybe the Yamaha tank is better.