In the Opus 18 Sextet in B-flat Major, Brahms has all the instruments playing all the time, so he keeps the violins in their upper registers so they can be distinguished from the other four.
In the Opus 36 Sextet in G Major, written seven years later, Brahms only has the instruments play when needed, so he doesn't have to push the violins into the upper reaches.
It's two very different solutions to the formal problem of sextet writing, and Brahms got it right on the second try.
Making all those strings work together does sound like a tricky problem. Glad he figured it out.
Publius.....#150!!