If you want to learn to juggle four balls, this is the first step. It is also used in many three ball tricks such as Columns & The Yo-Yo. Learn everything with both hands though. The title is pretty self explanatory apart from the 'inside, outside & columns' bit. These are the names of the three basic two-ball patterns that can be juggled with one hand.
In all three of these patterns try to keep both balls an equal width apart & only juggle in two dimensions, height & width, not depth.
Also referred to as 'rolling out'. This is the easiest one & is where the two balls go round in a circle; throws are made on the inside & catches on the outside, just like in a regular cascade. For the right hand the balls go round clockwise & when juggling with the left hand they go anticlockwise.
Juggling two in one hand is a little slower than the three ball Cascade, but only one hand is doing the work. Start with two balls in one hand & throw one ball to your eye level from the inside to the outside, wait until it peaks then throw the next ball.
Inside scoop is very important, you will need to scoop much more than when juggling a cascade. You will also have to let go of the ball later so that it is actually thrown to the outside & not to the other hand.
Imagine a pole sticking out of your bicep pointing out in front of you in line with your fore arm. The balls are thrown from the inside, over the pole, caught on the other side & then carried back under the pole by your arm's scooping action.
Also known as 'rolling in'. This time you use outside scoop (just like you would use for an Over the top throw) to throw the balls from the outside over the pole & caught on the inside. This is a little trickier because at first your natural arm movement will cause you to throw the ball further to the opposite side than you want to. All you have to do is learn to let go a little earlier in the scooping motion.
In columns the two balls do exactly that, travel up & down in columns. Like two parallel lifts, as one is going up the other is coming down. Imagining the pole again, one ball always stays on the left of the pole & the other on the right. Your hand will not carry either of the balls but will scoop back & forth using both inside & outside scoop in the shape of the letter 'U' under the pole.