I like the thought behind this, especially since I play both guitar and bass; though it would help if more of the instruments could be seen in silhouette, especially the curve of the bodies. My attention is mostly drawn towards the tuning pegs.
Due to remaining detail, I don't think those guitars are true silhouettes. I don't mind the colors, folds and shadows of the background, but that bright light is very harsh.
Besides not being a true silhouette, there's a fuzzy feeling here detracting from what should be a sharpness in the instruments' lines. About the title: try to avoid prepositions. Rhythm and Bass: Harmony's Combination might be more punchier. 3