My tanks are Teed as well with a manual shut off valve for each one. Manual valves almost ALWAYS work! A free benefit to a system like that is that it can give you a "reserve". Once I ran out of gas. Since my ass is fat and my girlfriends ass is not (yet), the tank on her side ran out before mine did. I just turned off the empty tank and had enough left in the other to get me to the ramp.
As for the fuel gauges they are about as accurate as weather forecasters. I use the gauge in my head.