Yes you have to open a new virtual instrument for each new melody you want.

Some virtual instruments are different.. you can use one virtual instrument, then “switch MIDI Ports”, but I usually just like virtual instruments where you right-click on the virtual instrument, go “clone”, then select a new sound, and you’re on your way!

So to repeat, you have to open a new virtual instrument for new sounds, then go to a new pattern, click on that virtual instrument, then you can play your new melodies, record, etc.