Change machine needle to twin needle.  Thread additional reel.  Try a sample. (Many machines have a second post to put an extra roll of thread on.  If yours does not, just place the extra roll of thread in a cup or bowl behind the machine.  Thread can still run off the reel easily from there, but your roll of thread won't bounce away onto the floor!  Ensure the foot you're using will allow the twin needle to go through without hitting and damaging it.)

Still mystified by twin needles? Your sewing machine manual may help or absolute beginners can watch this video from Angela Wolf.