You introduce two cubes, and give both of them to your audience to mix. Then you take one of them (either cube) without looking, and "solve" it behind your back so that the cube will match the other cube (the one they kept for themselves).
First of all you have to connect 2 cubes before you start the routine. Select "Cloned cube (2 cubes)" from the routine list, then tap on "Start" at the bottom.
You can hand out both cubes to mix. When you work with 2 cubes (in any routine), the cube that gets mixed (turned) first, will be the MASTER CUBE. It means when the mixing is done, the app will assume you will solve the master cube to match the other cube. So when the countdown is finished, the app will calculate the solving (cloning) steps for the master cube. After turning the initial U and R (orientating steps) you just have to follow the instructions to clone the other cube.
If you will solve the other cube (not the master one), the app will re-calculate the solving steps after the U and R turns. It means there will be a slight delay before you can solve the cube. Anyway, you can work with either cube, there should be no problem at all.
If the other (target) cube gets further mixed while you're solving (cloning) your cube, the app will add those extra steps at the end of the solving steps... so you will end up with a perfectly cloned cube every time.