A single sextuplet and a pair of triplets are similar in the fact that their rhythmic note values are equal, but are different (as affirmed in this post) in that usually a sextuplet should only have an accent on the first note, whereas a pair of triplets should have a slight accent on the first note of both triplets.
In measure 194 the second violins and violas are playing sextuplets (the first "two" of the "notes" are a quaver (eighth-note) rest, and the other four notes are semiquavers (sixteenth-notes). This pattern continues in measure 195 until the last beat. The last beat of m. 195 (marked in the above image) is a pair of triples, (the first of which is a semiquaver rest followed by a two semiquavers, and the second is three semiquavers).
I don't understand why the sextuplet grouping has changed to a pair of triplets.
I am not a string player, but as I understand it the notes under each slur (each beat) are to be played with a single bow stroke. I assume that this would make accenting the first note of the second triplet difficult/unlikely.
It there some subtle reason that makes triplets more correct or a better way to represent the notes in this beat? (perhaps it has something to do with the rests)
Is there some difference in the way these might be performed if the last beat was simply represented as another sextuplet (as opposed to the triplets)?