Does it simply add an unboosted extra shot like normal? Or is the second shot affected by the crank multiplier too?

Damage raised to near double per shot.
One sees only a single shot however.

does anyoe use these? I find them terrible due to the time it takes to reload

This doesn't matter so much when used as a sniper, especially as the damage it causes (especially with 6 cranks) more than makes up for the time taken.

Quick Hands perk is your friend

Note that you get an 6 crank one after old guns, mod can be moved to the unique you get then you meet Preston.

I didn't think they took longer to reload then usual, I'll have to try it as I actually like two shot for my sub-machine gun and sniper rifle.

I dropped these fairly early one (like the pipe guns). Other more useful guns appear so early on remembering to crank that lever became a pain. Would be good if there was an auto-crank mod (maybe there is one, never bothered with them after early play).

The two-shot weapons in general are kind of ridiculous. First, they do NOT expend two units of ammo. Second, each shot does not trace TWO projectile trajectories; either both hit the exact same part of the target or both missed. In effect, a "two-shot" weapon is actually a "double damage" weapon and nothing more.