is there any downfall from just using a brass Y from the the two taps on the tank and running to the carbs minus the second petcock?
You would have no reserve. And if you ever needed to turn off the fuel you would have to lift the tank and close the valve on the main petcock.
The only reason I was considering it was fuel starvation issues could be potentally solved this way with better fuel flow. I generally know when I need fuel, so the reserve is not a big issue for me. I was also considering keeping the second petcock and adding one of those vac powered lawnmower fuel pumps. Any thoughts on that?
You should not have any fuel starvation issues if running on Prime.
Are you sure the tank is venting properly?
Also, if your float valves ever clog/stick open, you will flood your engine with all the gas in the tank. Seems like a bad idea for many reasons.