Well yes strictly speaking, the same binary.
But in practice with Nix's default library resolution logic (the mentioned compiler wrapping) you will probably also have problems trying to build binaries in one package using version of a library (see note [1]). That is, I think you'd have to do some "hacking" to fix the issue, or separate the binaries into different packages (and maybe create a new package that "assembles" all the binaries by creating symlinks to them).
> Does NixOS allow to have both B-1.1 and B-1.2 installed, and A's different binaries to load the correct different versions of the library from B-1.1 and B-1.2?
Yes, well actually, the thing is that typically you only "install" (into a user environment) final applications (A) that you need not libraries, so the libraries (B) can be considered to not really be installed, just available (somewhere in /nix/store) and referenced correctly (e.g. by RPATH).
[1] When you declare a dependency in a package specification by putting it into buildInputs, the compiler wrapper will automatically add an -L/lib so that linking with just -l will find the library. With multiple versions of a library in buildInputs, that becomes a problem.