Pater has given some really good advice. It all depends on the texture/mood you're going for. Though you didn't ask for specific advice, I'll give you certain scenarios on what I might do with the passage above:
1. Have the melody in the 1sts, the triplet figure in the 2nds and violas (harmonized of course), with the bass line in the celli/bass. Rarely do basses and cellos do octave unison because it tends to overpower the bass line. You could do arco in cello and pizz. in bass if you don't want the bottom line to be too heavy, or just don't have the basses play.
2. Have the melody in 1sts and 2nds in octaves, though this would be better suited if you wanted mf or f in the melodic line. OR you can even give the melody to the 1sts and violas in octaves, with the 2nds doing the triplets.
3. Have the melody in the 1sts and celli in octaves, with the accompaniment in the 2nds and violas, and the bass line in the bass.
Overall, there are no general rules. You just have to experiment with different ranges and timbres.It would also be an advantage to know how many players are in the string section you are writing for. LOOK AT SCORES!!!! (Tchaikovsky, Dvorak, Elgar wrote serenades for string orchestras).