Alan,
Yes that is what I meant. The rollover valves are a part of the VENT system. They will allow air to move in and out as the fuel level changes as long as it isn't done rapidly. This keeps the tanks from pressurizing. The little ball in the valves will be pushed up as fluid rises in the tube eventually closing and causing back pressure(single vent scenario where the vent line isn't in the air bubble).
You can take one of these valves and hold it in your mouth and you can breathe in and out through the upright valve. But try to blow forcefully and it will shut.
The dual vent(per tank) setup is done with 1.) each line vented to air. They have to be high enough to allow fuel to run up the tube but not up to the rollover valve itself(slight overfilling of the tank is an example). Probably up to the level of the rear window or the sill in the front, in front of the windscreen. This is very expensive in that the roll over valves I found were not cheap. It would require 4 of them. 2.) Front and rear vent lines are joined above the level of the tanks near the top of the line and have one line for each side(what I have).
I guess there could be a third way where if you didn't mind running lines all over the car and terminate them all into the bottom of a "catch can" and using one rollover valve vented to air. A bit much I would think.
Micheal,
I didn't like the idea of having a lot of switches and mechanical valves to worry about. I am easily distracted and would wonder why the engine quit all of a sudden. Just more to go wrong(as you found out with your incident. I use one set of pumps, return line to the tank without the pumps and a link between the two tanks with a one way valve toward the pumped tank. The principle here is that the non pumped tank will flow back to the pumped tank when its volume is higher than that the pumped tank. If the return flow is faster than the connected line, the non pumped tank will fill til the rollover valve closes, and then fuel will be forced to the pumped tank(this would only happen if the linked tube is so small that flow to the pumped tank is very slow. I chose a large line and this has never happened.
What I do see is a small difference in the fuel level gauge(in the pumped tank). When running the level will be a little lower than when it is shut off. The only inconvenience here is that my low level warning lite will come on a little sooner than it should, which isn't really a bad thing. There is really a lot more to it in my car, but the principle is the idea here. If you want to know more, take a look at my build thread DRB #5 or PM me and I'll talk you through it.
If and when I get my car on the road(should be any day now if nothing else goes wrong) and have had a little fun with it, I plan to modify this setup in my car to be a little more complete and rule out any "possible" short comings of my system. It will involve pickups from both tanks and returns to both tanks with trap doors in both around the pickups and the ability to fill both tanks from one filler pipe.
Bill