In the updated Paragon Mount, the main difference remains Weapon-element skill and a Elecomp skill, this (dis)mount is an arbitrary indication of which skill is available on the skill menu and serves the purpose of restricting the menu to show one skill at a time instead of two.
As Kamui states in Tinkerer's info subs, its not double MC, rather a single MC that is compressing more than one item, we have existing equipment such as Moglord Tortress and Frogzard Sword from which Drakath's Fiend Blade is based on.
Griffin/Grave Mount were designed with the new concept of resource-free Elecomp attacks since the majority of Elecomp attacks are skills.
Its a design choice that Elecomp is applied to a normal attack or skill, the former can be used every turn due to no resource cost while the latter is dependent on resource availability, this limiting factor is removed for those who have access to Essence Orb, which was released after these two Mounts.
Its important to note that in chronology, questions that demand explanations for the lack of an earlier occurrence of concerned actions preceding a time of the concerned date is logically incoherent, therefore if an answer must be given, it was simply not thought of at that time.