The part is called the vapor separator.
The dual line set-up is called "Hot fuel handling"
It is designed to keep a supply of cool fuel available at the carburetor by returning fuel heated by high underbody temps back to the fuel tank, thus eliminating vapor lock as beetle pointed out!
The fuel pump produces 4.3 psi.
The vapor separator also reduces pressure to the carburetor by returning fuel to the tank.
If one plugs the return line this will over pressurize the carb needle/ seat assembly and possibly flood the carb.
Also it will deadhead the pump and prematurely wear it out!
Good luck!