Most cross-eyed people suppress the image of their inferior eye or alternate between eyes. The trick is to learn how to use them both at the same time in a correct way without suppressing either one or the other. One of the ways to do this is with a the red green 3D glasses and red green computer images so you have to use both eyes at the same time to see everything displayed on the screen. Most exercises like this are rather boring, you train the ability but it’s not much fun. Another blog I found proposes a red green card game making it rather fun to practice using both eyes together.
With the 3D glasses you will need both eyes, otherwise you are not seeing half of your playing field. I like this kind of creative and fun solutions to promote behavioral changes. It reminds me of the VW fun theory.