The universe of Berserk is made of three realms, each one having its own "physic" laws if studied independently.
We have seen for example that ghouls-ghosts of the astral realm could enter and posses physical bodies, granting them with new possibilities: seemingly more strength or for example the ability to do feats such as moving one's decapitated pieces at will. So it seems that in the astral realm -or the interstice- "laws" could respect more desire than matter.
In this sense, it's my belief that Guts was able to wield the DS because of this. This because it seems physically impossible for a human to accomplish such a feat. Besides this is made clear by Godo, creator of the DS. Eventually, the first time Guts wielded the DS was in such an emotional condition of rage, hate and destruction against it's apostle aggressor that wielding the DS against him, a seemingly two-handed sword (a concept which was principal in Guts' psyche as a means of destruction allready-as we have seen in Taka-no-Dan) able to destroy the apostle, could have made part of his primal and principal emotional desires of the moment - a desire giving him the necessary energy or power while in the interstice to accomplish the feat.
What I am saying here, is that Guts, if he weren't in the interstice first place, and if he have never had the desire to use the DS as his means to harm his enemies he most propably would not have been able to wield it against them.
We have seen for example that ghouls-ghosts of the astral realm could enter and posses physical bodies, granting them with new possibilities: seemingly more strength or for example the ability to do feats such as moving one's decapitated pieces at will. So it seems that in the astral realm -or the interstice- "laws" could respect more desire than matter.
In this sense, it's my belief that Guts was able to wield the DS because of this. This because it seems physically impossible for a human to accomplish such a feat. Besides this is made clear by Godo, creator of the DS. Eventually, the first time Guts wielded the DS was in such an emotional condition of rage, hate and destruction against it's apostle aggressor that wielding the DS against him, a seemingly two-handed sword (a concept which was principal in Guts' psyche as a means of destruction allready-as we have seen in Taka-no-Dan) able to destroy the apostle, could have made part of his primal and principal emotional desires of the moment - a desire giving him the necessary energy or power while in the interstice to accomplish the feat.
What I am saying here, is that Guts, if he weren't in the interstice first place, and if he have never had the desire to use the DS as his means to harm his enemies he most propably would not have been able to wield it against them.