I’m quite a bit more inclusive than others here in what I consider comprising a release for digital (YouTube/Soundcloud) artists. In this case, I’d probably add a single for the “unplugged” release if it’s the only place that version appears. In general, my rule of thumb is that if something is on a song-sharing site (as opposed to Bandcamp’s album focus) as “from our new album” or is presented as a music/lyric video, I’ll add the link to the track or the standalone video recording, respectively, but not create a release; on the other hand, if that is the primary way the artist distributes their music – uploading each track as they finish and especially if they rarely, if ever, compile them into albums – I will create many Single releases. As @luziferius said, the “unplugged” version is almost certainly a separate recording, and if that radio station’s series is anything like the “X Sessions” I’m familiar with, that’s practically the only place to find that version – I’d call it enough for a single.
As for how each are connected, you have a couple options depending on the exact nature of each. If they actually have the same sound and the “video” is just a still image, they can use the same recording. If only the former is true (ignoring sound effects), the video recording can be linked to the audio with the “music video” relationship. If they sound distinctly different, however, the recordings shouldn’t be linked directly, and instead should just be given either the same work, or two works linked via a “based on” relationship.