I’m not 100% clear on what you mean with “I know we cant do both at the same time”.
How FL Studio works is we create patterns, then we add those patterns into the FL Studio Playlist.
In the Channel Rack, you’ll load up a Virtual Instrument. (Let’s say Nexus, as I use that often in my courses).
On Pattern 1, you can right-click Nexus, then select Piano Roll. Any melodies you click in here will be within Pattern 1 on the Nexus Instrument (let’s say it’s a piano sound).
The way how I show beatmaking is then loading a new Virtual Instrument for each new instrument.
So I’d go to Pattern 2, open a new instance of Nexus, select a type of sound.. (let’s say guitar now), then right-click the second instance of Nexus, and go Piano Roll. Again, any melodies you program here will be on Pattern 2 within the second instance of Nexus (the guitar).
Usually best practice is thinking about patterns in terms of GROUPS.
I only want the piano to be pattern 1.. and I only want the guitar to be pattern 2..
This way I can add and remove those melodies within the Playlist at any time for much more control over the arrangement of the song.
To go further.. let’s say we go to Pattern 3, and want to make a drum loop.. let’s say we load in a kick drum, a clap, and hi-hat.. we can use all 3 sounds in 1 pattern if we’d like.. but just understand that it handcuffs you when it comes to the arrangement of adding/removing sounds within that pattern within the Playlist arrangement.
I hope that gives some clarity to your answer.. if not, I need you to be a little clearer on your melody programming in FL Studio process.