The phrase has to be two words of one syllable each, both beginning with the same letter. Using the same word twice is a no-no!
To start:
Big Bang
Good Grief!
Fast food
zig-zag
criss-cross
Lay low.
Black book.
Say so.
Tick tock.
Big boy
Ding dong
Tip top
Toy train
Hip Hop
Bull Shit.
That’s not alliterative, though.
Tough Titties
And that’s too many syllables. 
Nevermind
Cold-cock
Dead duck
Free-form
Bat Boy
ball boy
free fall
My bad. Shoulda read the OP more carefully.
Bill Board
Dammit! 
I even meant to write “tough tits” and still fucked it up.
Good catch, though.
Sad sack
Hell hole
big-boned
Done deal
Knick knack
hot head
dog day
tall tale / tell tale
work week
blue bell
blue balls / brass balls
nud nick
Loose Lips
Sink Ships