My small pit orchestra string section now stands at 3 violins and 2 celli. There are a few places in the score where dividing the violins (having only two of them play, in thirds and sixths) just sounds too thin, even though few other instruments are playing. I'm thinking about having all three violins on these parts now, each of them playing double stops. That should give me, roughly, as much weight as six violins in a 2-part divisi.
I know that six violins is still a very small string section. But, it does sound significantly better when I listen to the arrangement with my sample libraries.
I know that great violinists can play almost anything. But I need a few good "rules of thumb" about how much a decent violinist can play double stops (arco as well as tremelo) before the technical demands start to affect his performance. These are all slow, or moderately slow tempos, with fairly ordinary harmonies (as I said, thirds and sixths, mostly).
One of my orchestration books says double stops are playable on any two adjacent strings. That sounds overly broad to me. Doesn't the position of his hand on the neck of the instrument limit that range a bit? And it doesn't say anything about how fast a player can switch from one two-note "chord" to the next, if I am double stopping a whole line. Or how long a line a violinist can play double stopped, before he tires of it or it just becomes too difficult.
I don't think I'm asking for anything too demanding. But I'm not a violinist and I don't really know. I'm trying to make educated guesses. (I'm not afraid to guess, but I'd like to be better educated about what's possible and what's the norm for violinists in small pit orchestras for musical theater.)