I don’t want her to know we’re in cahoots.

cahoot (n) : partnership; league; as, to go in cahoot (or in cahoots) with a person; usually used in the plural, and in modern usage often used to imply that the joint effort is unethical, shady, questionable, or illegal; as, a shill in cahoots with a pickpocket, to serve as a distraction – source: Some Like It Hot R2 DVD, 37:07