First: Don't do that. Configuration files are expected to belong to only one package. Better use some configuration management.
If you really want to do it anyway, you likely need to use
dpkg-divert in the maintainer scripts (
postrm, etc.) to rename the original configuration file. See the maintainer scripts of grml's
grml-etc-core package for an example:
Once again: Despite
dpkg-divert is used above (for a live CD) to "overwrite" conffiles, it's not expected to work that way either, so expect that it may fail under some obscure conditions.