Due to the filler (there's more filler than compound for a 1mg cap) and random distribution of the compound within the capsule, it's incredibly difficult to get EXACTLY half (There may be 0.3mg of compound in one half and 0.7 in the other, even if both halves of the capsule are equally filled after separation) because you can't easily distinguish between the filler and compound without chemistry.
Just split the capsule and hope for the best. Look for tabs or smaller caps next time!