Merge from branch to trunk

Hi,

we have a trunk with our application and two branches with the same
application that has different database backends implemented (some files
are different).

I would like to keep the trunk and the two branches in sync except for those
files that are different (about ten files).

Ideally if trunk changes copy the changes to branch1 and branch2,
if branch1 changes copy the changes to trunk and branch2,
if branch2 changes copy the changes to trunk and branch1.
(except those files that will always be different).

Is it possible?
What is the recommended procedure?

Acording to the subversion book when I do:

svn merge --reintegrate http://svn.example.com/repos/calc/branches/my-calc-branch
in working copy of the trunk the branch is no longer usable for development.

Thanks.

--
Jiri Pejchal
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