[ploturo]

That depends on how correct you need it to be, since there is a workaround but also one issue specific to the double sling and some other bugs with double weapons and off-hand attacks in general.

If your Strength modifier is a positive even number like +2, +4, etc (for some reason, the sling doesn't round properly for the damage bonus on the offhand attack), you can get a mostly correct set of attacks and damage by using the Weapon Size adjustment to increase the size of the sling to a size above your hero (+1), and then checking the box to use it as a double weapon. Then you can turn off the weapon size adjustment to go back to the correctly sized weapon and the double weapon setting should stick.

Two weapon fighting and the improved version should work, but you will still have to deal with several issues that affect off-hand attacks in general (negative modifiers being halved for the off-hand instead of using the full penalty, hasted/speed effects or rapid shot incorrectly adding an additional attack to both primary and off-hand attacks).