Well I don't know what's going on here, since I have 6 Intimidate, 12 Lore and 6 Spellcraft, as well as Brimstone blast, but I can not pick the Hellfire Warlock class.
As a test I removed Brimstone Blast and took Hellfire Blast at the next level, that unlocked the class. Then as a test I removed Hellfire Blast and went to Brimstone, and the class was still unlocked.
So I decided to do the test again, and this time instead of Brimestone I took Hellfire. (New Character, new module so nothing of the old one left) That unlocked the class on the first go.
This is weird. Although you are correct in that you only need one.