Single vs duel fuel lines?

[cafeguy]

My tank only has a single fuel line running to both carbs, saw this the other day and was kinda curious as to why it has two lines? (as in i have 1 petcock the tank in the image has 2)

 

[weaselbeak]

It lets you get all the fuel from both sides of a tank. On some tanks a considerable amount of gas can be trapped on the off side because of the tunnel.

 

[650Skull]

Chicken or the egg..................All XS650's from 70 to 79 had BS38's and each carb had its own fuel feed, hence two petcocks. When they changed to BS34's on the 80 and later models, (single feed with a T between the carbs), they only needed the single petcock, tanks were made to suit the carbs