Immortal E can max or supermax both skills as well, but both Immortal A and Immortal E would need to supermax a skill that no other immortal has or will max or supermax.
For example, I had Immortal A Supermax Cooking as his unique supermaxed skill. That means that no other of my immortals can either max or supermax Cooking as cooking needs to be uniquely mastered/supermaxed by Immortal A for him to qualify for his ambrosia.
Immortal B supermaxed Fishing as his unique supermaxed skill. So, no other of my immortals can either master or supermax Fishing due to ambrosia requirements.
So forth and so on through the 8 generations. So, at the end you'll have 8 immortals each with one skill that they have mastered AND they have completed all of the skill challenges. This is what this forums refers to as Supermaxing. No other immortal can master or supermax these skills.
Then there are the other skills, that I think of as 'Free' skills. These are skills that any or all of the immortals can Master, Supermax, etc and this will have no effect at all on any of the immortals qualifying for ambrosia.
For example, I might have the following (all caps is my unique supermaxed skill qualifying that individual for immortality)
Generation 1 COOKING, Gardening, Guitar
Generation 2 FISHING, Gardening, Bass
Generation 3 PHOTOGRAPHY, Painting, Guitar
Generation 4 WRITING, Painting, Piano
Generation 5 INVENTING, Sculpting, Handiness, Painting, Bass
Generation 6 LOGIC, Athletics, Guitar
Generation 7 CHARISMA, Guitar
Generation 8 DRUMS, Guitar, Piano, Bass
Basically once you've decided on a supermax for one immortal, it is HANDS OFF for all other immortals. For me, I find that using a spreadsheet helps me work all this out ahead of time.