Can minimal distances be applied to dampers installed in a single opening sealed with hard stone wool boards?

Yes. The total opening in the wall to be sealed with hard stone wool boards is subject to two restrictions: the maximum size of the opening should be equal to or lower than the damper sizes + 600 mm AND equal to or lower than the maximum allowed blank seal (m²) as defined by the stone wool board manufacturer (for ex. Promat WxH ≤ 3,75 m², Hilti EI90S ≤ 3,20 m², Hilti EI120S ≤ 1,44 m²).