Description of problem:

Compare compat-db46 in F12 and F13:
compat-db46-0:4.6.21-11.fc12.x86_64
compat-db46-0:4.6.21-3.fc13.x86_64
                     ^
The compat-db46 release value in F13 is less than the release value used. But you need to ensure that the release value for F13 is greater than the release value in F12. The easiest way might be to now bump the release to 11.%{?dist} in F13 and then only increase the release value in F12 after the %{?dist}, e.g. the next release value for F12 would be:

11%{?dist}.1

Another option would be to not use the same release value for the dbXX subpackages that the main package uses, but e.g.

%global db46_release 11
%global main_release 3

Main package:
Release: %{main_release}%{?dist}
Requires: compat-db46%{?_isa} = %{db46_version}-%{db46_release}%{?dist}.%{main_release}

sb46 Sub package:
Release: %{db46_release}%{?dist}.%{main_release}