'Final Form Elements' are their name, they are activated when your fighter reaches it's 'Final Form', i.e. a Super Rare fused all the way until it turns into an Ultra Rare, then the second element will be activated. I think that it also is activated when the fighter reaches it's maximum level, but that needs confirmation. Benefits of having 2 elements are that you will do extra damage to 2 elements instead of one, disadvantages are that other fighters will have twice the elements to counter you with. I don't think it's really a benefit/disadvantage thing, more just the type of fighter and aesthetic really. =]
A FANDOM User
A fighter does not get its second element activated by leveling it to max level (confirmed with currently lvl 40 R+ experiment 003).