It's fine either way. I just misinterpreted it to mean that by using those Seals once, said class is added to the Heart Seal set.
For example, I paired Takumi with Oboro. Takumi would normally inherit Spear Fighter from her, but that's his secondary so he should have gotten Apothecary, but instead no new classes were added. Or Leo, who married Felicia and should have gotten Maid, but that's his secondary so he should have inherited Hero. Again nothing new.
My thoughts are starting to go towards it being you only get the secondary if the first matches other units first. In which case I really screwed up on a lot of my A+ ranks because I ignored any where classes were the same when in fact could have been quite useful.
This is what all my experience has told me, it's just on many sites, the Fire Emblem Wiki included, it mentions being able to reclass into the Avatar's secondary.