What is a "shadow game" from Gone in 60 Seconds?

In context he seems to be referring to using misdirection to throw the police off their scent, presumably boosting cars they don't need and moving from area to area in order to force the police to try to work out their (heavily disguised) plan.

For example, If one wanted to steal ten Lamborghini cars to order, perhaps stealing three the first night along with six (unneeded) Mercedes cars would confuse law enforcement:

"You got maybe a better plan?" Donny Astricky asked, completely skeptical. "Yeah!" Kip said. "You spread it out. You move around. Shadow games and shit."
"Right," Memphis said, dismissing the comment. "Go on, Otto."
"Since we're on a truncated timetable, we take a day to shop it, a day to prep, and—"
"Hey," Kip interrupted. "Didn't you hear what I said?"
"Yes. We heard," Memphis said. "Shadow games. But what you have to understand, Kip, is that if you play shadow games, by the next night, the heat is on to you. With a one-night boost, by the time the first cars are reported stolen, your ship's set sail."
Kip and the others nodded begrudgingly. It made sense. Memphis nodded to Otto, who continued his analysis. - _{^{Gone in 60 Seconds: Movie Tie-in}}

The term "Shadow Games" is associated with the term "Cloak & Dagger", and is usually used in connection to some sort of spy or espionage-type activity. Misdirection is a key component, and (as any James Bond fan could tell you) it may involve making one action appear to be a different action.

In this context, Kip is suggesting stealing one car from one area of town, another car from another area or surrounding town, etc..., not on the same night. Spread it out. Don't let anyone recognize there's a pattern. Case out the cars (which they do for one Mercedes, actually), make careful decisions on what the easiest targets are.

Memphis, on the other hand, suggests the polar opposite; get them all on the same night, as quickly as possible, before information can be exchanged by law enforcement. Grab and go, and let your driving skills get you out of trouble.

Shadow games isn't stealing extra unneeded cars. It simply means to spread out the time of stealing. Steal 25 tonight...25 tomorrow night. Or skip a few days if you have time to do so (which they didn't at the moment). Prime example: remember at the beginning of the movie when the warehouse got raided. Kip and his crew had only collected 13 cars and the order had already been placed for the cars two weeks ahead. Kip was stretching out the boost...playing shadow games...But being hella slow at it haha..