When you use the longsword with a shield, you also use your 1h proficiency. But, you also get a 35% penalty to damage and swing speed, which is a HUGE difference. These are the stats when used as a 2hander:
speed rating: 98
thrust damage: 23 pierce
swing damage: 36 cut
These are the stats when used as a 1hander: (The red marks the comparison with the 2handed stats)
Speed rating: 64 -34
Thrust damage: 15 pierce -8
Swing damage: 23 cut -13
In comparison, lets look at this 1hander, the Simple sword
speed rating: 99
thrust damage: 23 pierce
swing damage: 27 cut
This is the worst real sword there is among onehanded weapons, and its still A LOT better than the Longsword when used in 1handmode. Furthermore, the price on the Longsword is simply outrageous in comparison to its speed and damage when used with a shield.
Conclusion: Use the Longsword primarily without a shield, as otherwise it is a clumsy, non-lethal and expensive weapon.
One more thing though: While wielding your Longsword in 2handmode, you can press "x" to chance the mode of the weapon. This will make you use your polearm wpf on the Longsword, and also slightly change the stats (More damage on stabs, less on swings and slightly lower speed, I think). You will also change the animations with your weapon, so that you use a polearm animation when you stab and using the left-to-right swing. You will also not have any overhead animation, but still have the 2hand animation on your right-to left swing. You can easily switch back to using your Longsword with 2hand proficiency by pressing "x" again.
Hope this clears it up.