I played with this concept before. A champion that utilizes different weapons. I am very well aware that giving it a second set of abilities would be boring, since we have enough of such champions. However, most of them are not proper multi-wielders. Jayce has a hammer and shock rifle, but it's basically the same thing on two different modes.
I propose a bladesman - archer duality.
Naturally, a melee attacker, wielding a pair of blades, uses his abilities to call on a shadow assistant/alter ego to wield a bow and arrow.
(Innate): Enemies whose movement speed is impaired are marked for 3 seconds. This champions basic attacks consume the mark, granting bonus attack speed for a short time and a burst of movement speed. The mark can be consumed indefinetely, but the attack speed doesn't stack.
Q: Fires an arrow to a target location that explodes scattering more arrows in multiple directions dealing damage. If it hits a enemy champion beforehand, it will stop and deal increased damage and also cause the enemy to bleed for 60% of the initial damage.
W: Dodges all spells over a 0.8 second duration, maneuvering gracefully through the air and avoiding unit collision. If at least 1 spell is dodged, the next attack will make this champion dash to the enemy and deal increased damage. Damage is increased drastically at very close ranges.
E: For a short time, this champion gains % bonus of his lifesteal equal to his current % crit strike chance on his spells and non-crit attacks.
Example, if it has 60% crit strike chance, and 22% lifesteal, it would gain an additional 13.2% lifesteal, equaling 35.2% total lifesteal on his non-crits. Crit's apply the unmodded lifesteal of 22%. Also, his spells heal for 13.2%.
There are not many kits around managing critical strike chance. There is either Tryndamere, and Gangplank, and they are used mostly for damage.
R: Fires a long range arrow to a location. The arrow leaves behind a trail of smoke that lasts for 3 seconds and allows this champion to move stealthily (like in a brush, attacking will reveal this champion, and enemies inside can see them). When the arrow reaches it's destination, it will leave behind a shadow archer whose attacks' range reduces to 625 over the first 2 seconds from the initial range. Initial attack range can be a maximum of 1500, and depends on how close this champion was to the shadow archer at the time it spawned. Shadow archers range can reduce even faster if this champion moves close to it quickly. Shadow archer's range doesn't go up if this champion moves away. This archer's attacks apply on-hit effects, lifesteal and can crit (gains 50% of total critical strike chance and is calculated separately), but gains reduced attack damage, has fixed attack speed and takes increased damage from enemy champions' basic attacks.
Bonuses from passive and E apply to this unit aswell, increasing it's attack speed and healing the champion.
The shadow ill move in narrow circle around the initial location for a better shot.