The PA28 with an entirely pumped fuel system (no gravity feed unlike, say, a C172) can't be allowed to draw fuel from both tanks at once because if you get to the point that one is empty and the other part full, the pump will try and draw the least viscous fluid available - air. So, you'll get a fuel starvation engine failure whilst there's still fuel remaining.
So, only gravity fed systems are allowed to draw from multiple tanks at once.
UV - I'm fascinated by your statement that you are looking for maximum 2 minutes from a fuel system after shutting the fuel off. I can imagine a type specific minimum for something aerobatic that deliberately uses a large fuel line capacity to ensure a continuous fuel supply whilst in negative g - the Bulldog is like that for example. However, I'm not aware of any airworthiness standard that mandates such a thing, notwithstanding that what you describe seems quite sensible.
G