Two is enough for a secret.
( Two ( ( is ( enough ( for ( a secret ) ) ) ) . ) )
(ROOT (S (NP (CD Two)) (VP (VBZ is) (ADJP (JJ enough) (PP (IN for) (NP (DT a) (NN secret))))) (. .)))
A secret needs two people. 
( ( A secret ) ( ( needs ( two people ) ) . ) )
(ROOT (S (NP (DT A) (NN secret)) (VP (VBZ needs) (NP (CD two) (NNS people))) (. .)))