This might be a stupid question, but everytime I use my whoopie slings, I can't help wondering why whoopie slings are actually slings? Is there any reason not to use 2 individual pieces of cord? Would that make the suspension too weak?

It's just that you basically need double the amount of rope with a sling than if you would when using just 2 pieces of rope. Also I would be tempted to fix the whoopie sling to the hammock the other way round, so that the part where you adjust the length doesn't move away from the hammock. If you have to hang your hammock between trees that are spaced far apart, it can get a bit tricky for a small person to get to the adjustable part...