Merging problem

I have a repository with directories A and B.

I want to merge A is a repository revision 2 and B is at repository
revision 4.

I want to checkout A, then merge A and B into the working copy.

How do you I do this with TortoiseSVN? (or SVN for that matter)

Every time I try to use the merge function, it ends up deleting the only
directory under either A or B, then just adding all of the files and
directories for entire B directory.

B is the latest vendor's version of the software.

A is my version that I started modifying based on the last release from
the vendor.

Thanks,

Thomas Sturgeon

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