Comment 13 for bug 403291

>Please take care that both of you add
> Provides: ubuntu-derivative-default-settings
> Conflicts: ubuntu-derivative-default-settings
>to avoid settings fighting each other and file-conflicting over default.session.

I'm sorry, I don't think that is an acceptable workaround. Take a look at what happens on a system with both mythbuntu-desktop and xubuntu-desktop installed:

satyr@satyr:~$ sudo aptitude -f install
[...]
The following packages are BROKEN:
  xubuntu-desktop
[...]
The following packages will be REMOVED:
  xubuntu-default-settings{p}
[...]
  xubuntu-desktop: Depends: xubuntu-default-settings but it is not installable
The following actions will resolve these dependencies:

Remove the following packages:
xubuntu-desktop
[...]
satyr@satyr:~$ sudo aptitude -f install xubuntu-default-settings+M
[...]
The following packages are BROKEN:
  mythbuntu-desktop
The following packages will be REMOVED:
  mythbuntu-default-settings{a}
The following packages will be upgraded:
  xubuntu-default-settings
[...]
  mythbuntu-desktop: Depends: mythbuntu-default-settings but it is not installable
The following actions will resolve these dependencies:

Remove the following packages:
mythbuntu-desktop

satyr@satyr:~$ aptitude why xubuntu-default-settings
iB xubuntu-desktop Depends xubuntu-default-settings
satyr@satyr:~$ aptitude why mythbuntu-default-settings
i mythbuntu-desktop Depends mythbuntu-default-settings

I think, if you're going to take this route, that the Depends of both packages need to weaken their Depend into a Recommends. That will probably bring a host of other problems with it, but it is safer than breaking installations, imho.