Arthur is basically right for some guitar, and Daniel tries hard.
To achieve a good guitar representation of both harmony with a melody, finger-style is the method of which to best interpret any piece for a single player. Having to know the chords, progression (left hand, at the fingerboard) and the notes to play at the same time (right hand, over the strings), can then be accomplished.
But, just picking each individual string and in sequence (with the right tempo and rhythm pattern) with a pick, or cross-picking, will as well do the trick. As true finger style is advanced (free from a plectrum) and can ad more color and expression to any selection.
For piano, the physical relationship is there as well.
The left hand provides the accompaniment (piano key chords) of harmony, a bass line and sets a tempo or rhythm pattern for the music being played. As the right hand then follows the melody, or notational guide of any song at the same time.
The real challenge in both guitar, and piano playing situations, will be to get the coordination down and to learn to have both hands in a comfortable synchronization (working together) or unison over time, to then provide a complete and recognizable song pattern.
Once you come to understand and use these methods through exercises, continued practice and dedication. It will be much like when first learning to swim, and knowing you're not going to ever again sink.
Learn to float and tread water first. Then, go for the length of a pool.