One choice we have is having two attributes at 5 (split key attributes), instead of one at 6 and the rest at maximum 4.
If you house-rule it - I have no problem with that.
The way I read the rules, though, it sounds more like this: All characters have attributes rated from 2 to 4. Only one attribute may be above 4, if it is the key attribute to your Kin or Profession. If the attribute is key to both kin and profession, it can be a 6. Otherwise it may only be a 5. But still only one attribute above 4.
I will run my campaign with this interpretation. If my players want to make characters with two attributes at 5, then they will have to find another GM.
"Whenever I encounter a problem I can't solve, I throw a molotov-cocktail at it, and then I have a NEW problem instead."