GuitarBizarre

I don't really consider playing instruments that are too similar, to be the same thing as playing two instruments. A guitar player might not capture the nuance, but he's innately capable of playing banjo, bass, ukelele, and mandolin very easily.

On the other hand, a guitar player that can play Trumpet, Violin, Piano, flute, and Saxophone is capable of playing practically any non-percussion instrument that exists, because he has a very wide skillset.

Edit: Also, Saxophone and Cello are very different instruments. The issue isn't the sustaining notes to me, its the lack of a wider ability. that informs the practical aspect of achieving those sounds. A Cello player cannot approach a saxophone with any measure of familiarity just because of its ability to sustain notes. Its absurd to consider the two things as having a similar approach.

Its far more reasonable to consider the physical act of playing as the defining similarity. IE, fretted instruments, reed instruments, brass mouthpieces, keys, fretless instruments, etc.