The problem here is ABI, not API. Those two packages are the same source code, linked to different libstdc++s. They have all the same files, export the same symbols, etc.
Having them parallel installed would do really, er, interesting things when packages built against one version of libstdc++ try to link to libraries built against another version of libstdc++.
The c2 suffix is for the c++ ABI '2', alos sometimes called 1001. You'll see some packages in sarge with a suffix c102 - these are remnants of the last transition.