You wanted a guitar+mandolin combo.
Duplicate the guitar track , (press Ctrl + D).
Apply “Sliding time scale/ pitch shift” , (in “Effects” menu), with settings shown to the duplicated guitar track.
Voila you have guitar and mandolin (ish) …
Applying some randomization to the amplitude and timing would make it sound more like two humans playing,
there are two plugins which randomize …
Missing features - Audacity Support *
[ * Random_Pitch_Modulation applied to a stereo track makes the instrument wander about in the stereo field ].