The only real difference between a scabbard that hangs from your waist and one that hangs from your back is the direction that the scabbard aligns with the belt.
For the waist it is about halfway from being perpendicular to the belt. A shoulder scabbard would be parallel with the baldric strap across the back.
A sword that is longer than your arm can be drawn from the back if the scabbard is the type with a slit running along part of the edge(forget what that type of scabbard is called), or what Ambrosia shows in the second link below. The slit/space should be long enough to compensate for the sword length versus your arm length, but not too long that it's in danger of falling out when the wielder starts running.
Using two swords simultaneously is a particularly difficult thing to do, and there is very little advantage, if any, over using one sword of an equal size. Saw a martial arts show once that demonstrated that, can't really remember it anymore though.
The Japanese sword art of Hyōhō Niten Ichi-ryū used two blades, although that was with a katana and a wakizashi (which is shorter).