If you look at the screenshot you originally attached, the two voices shown are the first up-stem and down-stem notes on the first staff – the second (new) staff doesn’t show at all. This is because the display in Play mode doesn’t currently update as soon as new voices are added, which is not great: you can find details about this here:
I don’t have VSL on my Mac here, but I find that it I just swap the VST instruments in the rack for e.g. Pianoteq, each of the staves does play back as expected. Unfortunately I don’t think I can offer much help with why the “Solo” playing technique isn’t having the expected effect – but perhaps another VSL user can assist you in that area.