I agree with the post above. Take this:
10 knights. you get 5 extra attacks with the proposed second rank with lances attacking. Doesn't sound broken, and 11 attacks is hardly game breaking.
Now try 12 chaos knights 6 wide. suddenly you have 25 s6 attacks from a unit that should cost a lot more for that ability. What if they're frenzied? 37 s6 attacks is not balanced.
Lets take this to the extreme. 12 Blood Knights. 6 wide. do you really want 49 attacks at s7 and KB to hit anything? EVER? that unit would blast through anything in the game. It doesn't matter if it's Deamons or whatever. Try and think of something ranked.
that unit will hit 32.34 times, and wound 26.8 times. 4.4 of those will be KB. Lets see you try.