That is awesome! Although I imagine rendering the geometry for two simulated actors would be no small feat, considering the arbitrary angles they could be attached at.
And you’re right about Flex, I looked into it and it’s just the UE4 plugin that is open source as far as I can tell. It would definitely not be able to be integrated into the VICO physics library as a whole, just the VD UE4 plugin (and maybe not even that? I don’t know how much they’ve exposed in their plugin). I can definitely see why that would be undesirable.