We're going to have the upper octave begin one bar later than the lower octave. Another term used for this, especially when it's a group of people singing, is 'in a round.' This step will be to have the melody play in canon.
Now I'll undo that velocity change because I'm going to want equal velocities in this next technique. To do this, change the Color Group Selection in the upper-left to green, use keyboard shortcut Shift+C to select by color, and then use keyboard shortcut Alt+X for the Scale Levels tool. As an example, let's lower the velocity of only the green group. For example, if we wanted to apply different quantization to one of the groups or if we wanted to lower the velocity or timing of one the groups, we could easily do that. The main thing we've done here is to make it easier to select groups of notes in FLStudio. With the new notes selected, change the color group selection (which is the pallet of colors in the upper left) and use keyboard shortcut Alt+C (think ALTer + Color). In order to keep track of the higher octave, let's change the color of the new piano roll notes. This isn't the sort of thing you'd normally do with a piano part, but more likely with an ensemble of strings or brass. Next, I am going to make a copy of the melody and have it play one octave higher, in addition to the melody already in place.
Let's begin with a piano melody, I recorded it in FLStudio with my midi keyboard.