What precisely does this notation mean in the first bar treble clef? From ear, the note appears to be held until it’s sounded again - is that the intention, and wouldn’t a dotted eight be a reasonable alternative? Besides that, why are the two tenuto notes beamed?
It’s from Michael Nyman’s ‘The Heart Asks Pleasure First’.
The two tenuto notes constitute a separate "voice" from the sixteenth notes. It's as if two instruments are playing: the melody instrument plays the two dotted eighths; an accompaniment instrument plays the sixteenths. The tenuto notes are barred together for clarity that they are "connected".
Tenuto means as you say: give each tenuto note its full value. The marking is technically redundant; it's just there to reinforce that the note should not be released early.
Tenuto also often serves as a "soft accent", which is appropriate in this context to "bring out" the melody.
This post may also be of use: How does one maintain voice integrity when longer and shorter notes of the same pitch occur in two voices
The currently accepted answer isn't really correct.
Unaccented notes typically have a slight separation. The tenuto mark indicates that the note should be sustained longer than typical, reducing this separation. It's not a redundant marking. Keep in mind, the exact interpretation can vary depending on composer/style/era, but this is a good starting point.
Tenuto indicating emphasis is accurate, especially in this context as the line is in duple on top of a triplet rhythm structure. The composer wants you to emphasize the rhythmic tension between the upper and lower staves. (an example of hemiola) This is supported by the phrase markings and other accents.
Regarding the beams: beams do not tell you how to play the music stylistically, rather they indicate and clarify the rhythm in context of the rhythmic structure of the music. In this example, the music is in 6/8. So you typically "feel" each measure as two beats with a triplet subdivision. The beams here help indicate which notes are part of the same beat, and where the next beat starts. Eighth notes would be beamed in groups of three, sixteenth notes in groups of six. Contrast with something written in 4/4, which would have eighth notes beamed in groups of two, and sixteenth notes in groups of four.
If you see beams crossing the boundary between beats, that's generally an example of poor engraving, because it obfuscates the rhythmic structure.
This beaming technique is particularly useful in your example, as the top line here isn't in the same triplet feel that 6/8 would indicate. The beat is subdivided into two dotted eighths, (a duple feel) and the beams make it more obvious how that lines up with the rhythm structure. (two notes in count 1, two notes in count 2)
