I think if you use dynamic linking, it is in principle possible for module A to use v1 internally, and for module C to use v2, and neither to know about the oth

I think if you use dynamic linking, it is in principle possible for module A to use v1 internally, and for module C to use v2, and neither to know about the other. But you can't use global objects, and you can't share objects from the internal libraries between modules. And the dynamic linker on Linux at least tries to reuse symbols, so it doesn't work by default. This was a big problem in the Gtk2 / Gtk3 transition, where you could not use both in the same process. I think there is a flag to prevent symbol reuse, though.