Just wanted to share my exploration into this topic:
In short, the endpoint needs to contain both playback violins, and the way to do it is by using the button that is currently on the bottom of the VST list, not the “Save endpoint” button in the playback instrument entry. I was not aware of this distinction, so maybe someone else could use this pointed out.
This means that a “Strings endpoint” is the way to go, as far as I can see, in such a case as this. I guess the OP have had the problem solved by now, but at least I, for my part, found this thread a few years after its moment in the spotlight.