I seconds Tim's waryness! Merging is a *itch, especially if you have two teams making independent changes to the same source base. A "svn merge" will only do so much, and you will have to manually resolve any conflicts. I hope you have really good
unit and integration tests!
I would run a "svn diff" first. Or I would check out both branches and use some tool such as windiff, ediff or SynchronizeIt to view the differences. I would also compare the branches against the original. All of this comparing would help me better plan for the merging of the branches - I would at least know which files were going to cause me the most problems. It is hard to give specific steps because it all really depends on what was changed.
Good luck!