If you break down the character
「伙」= 人+火 suggest "people gather around cooking fire"
「夥」= 果+多 suggest " many fruit in one basket"
You can see 伙 is more intimate and voluntary act than 夥
夥同某人 may be just for convent, 伙同某人 suggest a tighter bond.
Also, 「夥」 in 「夥脅」 is the " together" part and 「脅」is the "forcing / threaten~ into" part. That make sense, we can't replace 夥 with 伙 in this phrase because「伙脅」doesn't make sense.
「伙」= 人+火 suggest "people gather around cooking fire"
「夥」= 果+多 suggest " many fruit in one basket"
You can see 伙 is more intimate and voluntary act than 夥
夥同某人 may be just for convent, 伙同某人 suggest a tighter bond.
Also, 「夥」 in 「夥脅」 is the " together" part and 「脅」is the "forcing / threaten~ into" part. That make sense, we can't replace 夥 with 伙 in this phrase because「伙脅」doesn't make sense.