The "self-priming" equalizing line in your bottom picture won't work. It can't self prime unless the level in one tank is a bit above the top of the line.
Unless the equalizing line is always below the bottom of the tanks. And even then, the draw will tend to empty one tank over the other unless the line length and everything else that causes restriction in the draw lines is equal (which isn't going to happen.) I think you will need a selector valve. I built two saddle tanks for a '72 Chevy PU I had and used a 3-way selector valve. Really pretty easy. Unless you forget to refill the inactive tanks and switch from a tank that runs dry to another empty tank.
But on your vent line, and easy way to deal with that is to connect a tube between the two fill pipes that's above the level of gas in the tanks.
