Fuel level twin sender

I’ve got a twin fuel sender fuel tank bmw e36, I have wired it up like the following image.

Fuel sender 1

I can see in the adu that it shows upto 3.7v, When I pulled the fuel pump / level sensor out of the side the filler neck is it’s full to the top, so it makes sense that it’s reading 3.7v. But how do I get the other side to work?

My values are

1.07 empty
1.52 5lt
1.98 10lt
2.32 15lt
2.61 20lt
2.84 25lt
3.02 30lt
3.06 35lt
3.06 40lt

As you can see at 30lt, this must be when the side is full up and flows over to the other side.

The pumps are in theory wired in series, although the original gauge has 2 lives 1 earth so I guess it works out that one side is full aka half a tank and then the other side voltage is added to the gauge to continue half to full?