There are quite a few very excellent bass players (mostly in the jazz world, I would assume) who play BOTH the electric bass guitar and the acoustic standup bull fiddle.

Observing their choice of instrument for any particular selection is interesting, and underscores the notion that the elecric and acoustic basses are, at least in some sense, different instruments.

On the other hand, there's a sense in which the bass guitar and the bass fiddle are the same instrument, and the washtub/tea-chest/gutbucket is another variant as well, along with that hybrid no one has mentioned yet (or have they?), the standup electric bass with a small solid-wood body standing up on an extra-long peg. All basses, all playing the same role in a musical ensemble.

You could even include bass horns (tuba/sousaphone). The stylistic line between the sousaphone and the electric bass has been muddied in recent years by New Orleans brass-band "sousaphunk" virtuosos like Kirk Joseph, who combine triple-tongueing technique with electronic amplification to make those big wraparound horns sound just like electric bass guitars when it suits their purposes.

There are even a few players who double on string bass (electric and/or acoustic) and tuba. Matt Perrine is perhaps the best-known/ most-recorded example of this breed.