Green’s Dictionary of Slang

(US/W.I.) a best friend.

1967		C. Cooper Jr Farm (1968) 170: All the acey-doosies and heavytalkers.
1992		Bentley & Corbett Prison Sl. 38: Ace-Deuce also Acey-Deucey Old prison expressions used to signify best friends.
1996		E. Bunker Dog Eat Dog 56: ‘How’re you and the old mafioso getting along?’ ‘We’re ace deuce. I beat the shit out of who he says, and he gives me money for it’.
2001		(con. 1975–6) E. Little Steel Toes 109: So do your ace deuce, cross-eyed road dog a favor, keep a low profile with your city ways.